.- 
I 


STEAM  ENGINES 


"Ms  Qraw'3/ill  Book  &.  7m 

PUBLISHERS     OF     BOOKS      F  O  P^/ 

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ENGINEERING  EDUCATION  SERIES 


STEAM  ENGINES 


PREPARED  IN  THE 

EXTENSION  DIVISION  OF 
THE  UNIVERSITY  OF  WISCONSIN 

BY 

E.  M.  SHEALY 

ASSOCIATE   PROFESSOR  OP  STEAM  ENGINEERING 
THE   UNIVERSITY   OF  WISCONSIN 


FIRST  EDITION 
THIRD  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC 

239  WEST  39TH  STREET.    NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD. 

6  &  8  BOUVERIE  ST.,  E.  C. 

1919 


Engineering 
Library 


COPYRIGHT,  1919,  BY  THE 
MCGRAW-HILL  BOOK  Co.,  INC. 


MAl-l,  K     PKKSS     YORK    V M. 


PREFACE 

This  book  on  Steam  Engines  was  written  to  be  used  as  a  text- 
book for  correspondence  students  in  the  University  of  Wisconsin 
Extension  Division.  It  is  the  third  of  a  series  of  three  textbooks 
designed  for  those  students  who  are  pursuing  a  general  course  in 
Steam  Engineering,  the  other  two  being  "Steam  Boilers"  and 
"Heat." 

In  this  course  in  Steam  Engines  we  aim  to  teach  the  fundamen- 
tal principles  underlying  the  operation  of  the  steam  engine  and 
to  do  this  in  as  simple  and  nonmathematical  a  manner  as  possi- 
ble. This  is  particularly  true  with  those  parts  which  deal  with 
thermodynamic  principles.  Enough  of  the  practical  features 
of  steam  engine  operation  has  been  given  to  illustrate  the  princi- 
ples, and  it  is  hoped  that  operating  engineers  who  take  this 
course  will  be  able  to  supplement  from  their  own  experience 
other  applications  of  the  principles  presented. 

That  part  of  the  course  dealing  with  valve  gears  has  been  made 
more  complete  than  other  sections  because  our  experience  shows 
that  operating  engineers  usually  do  not  understand  the  valve 
gear  mechanism  of  their  engines  as  well  as  they  do  other  parts. 

Most  of  the  material  in  the  chapter  on  Lubrication  was  fur- 
nished by  Mr.  R.  P.  Tobin,  Chief  of  the  Technical  Department 
of  the  Vacuum  Oil  Company  and  we  take  this  opportunity  to 
express  our  thanks  for  his  aid.  We  wish  to  take  this  opportunity 
also  to  thank  Mr.  J.  C.  White,  Chief  Operating  Engineer  for  the 
State  of  Wisconsin  for  very  valuable  suggestions  as  to  the  scope 
of  the  course  and  the  outline  to  be  followed,  also  for  many  useful 
hints  and  suggestions  about  writing  the  course,  and  for  a  care- 
ful and  critical  reading  of  the  manuscript. 

E.  M.  SHEALY. 

MADISON,  Wis., 
November  12,  1918. 


CONSENTS 

CHAPTER  I 
PRINCIPLES  OF  THE  STEAM  ENGINE 

ARTICLE 

Elementary  Principles 1 

Parts  of  the  Steam  Engine 5 

Classification  of  Engines 6 

The  Plain  Slide  Valve  Engine 8 

Speed  Regulation 11 

Automatic  High  Speed  Engines 13 

CHAPTER  II 

CORLISS  AND  OTHER  ENGINES 

Corliss  Engines 17 

Nonreleasing  Corliss  Engine      23 

The  Locomotive '..... 25 

Marine  Engines 26 

CHAPTER  III 

PARTS  OF  THE  STEAM  ENGINE 

The  Frame 27 

The  Cylinder 30 

The  Piston 37 

Stuffing  Box •  40 

The  Crosshead 42 

Connecting  Rods 45 

Crank  and  Crank  Pin 47 

Bearings 48 

The  Flywheel 51 

CHAPTER  IV 
HEAT,  WORK,  AND  PRESSURE 

Force 53 

Work 54 

Energy 54 

Heat 55 

Temperature .........  57 

Unit  of  Heat •    ••-   •    •"'•'.•    •  57 

Mechanical  Equivalent  of  Heat .'.... 58 

Specific  Heat    .    . •    ..,.•••    •  58 

vii 


viii  CONTENTS 

ARTICLE  PAGE 

Power 58 

Atmospheric  Pressure 59 

Vacuum 59 

Barometer 60 

Absolute  and  Gage  Pressures 60 

Measuring  Vacuum 61 

CHAPTER  V 

PROPERTIES  OF  STEAM 

Formation  of  Steam 64 

Interpolation  from  Tables 68 

Wet  Steam .    .    .   ,    .    .    .    .   .    .•  .    . 69 

Superheated  Steam ....    .  70 

CHAPTER  VI 
INDICATORS 

Work  Diagrams ..- 79 

The  Indicator .•...'    ...  ';    . 80 

Reducing  Motions 87 

Indicator  Diagrams 89 

Expansion  of  Steam    .    .    .    . " 93 

Ratio  of  Expansion ; 94 

CHAPTER  VII 

INDICATED  AND  BRAKE  HORSEPOWER 

Mean  Effective  Pressure    .    .  - 98 

Indicated  Horsepower 102 

Engine  Constant 103 

Brake  Horsepower 103 

Mechanical  Efficiency • 106 

CHAPTER  VIII 
ACTION  OF  STEAM  IN  THE  CYLINDER 

Cylinder  Condensation 107 

The  Uniflow  Engine 112 

Measuring  Cylinder  Condensation 114 

CHAPTER  IX 
STEAM  ENGINE  TESTING 

Principles .    .    .  ".    .    ;   .  ".    ,    .  118 

Steam  Consumption ,    .    .    .    .    .    .    .    .    ,    .    .    .  119 

Steam  Consumption  from  Diagram .    .    .    .    .    .    .    .    ,    .    .    .    .    .    .  119 

Duration  of  Engine  Test '  .    .    .......  122 

Efficiency  of  Steam  Engines .    .    .    .    .    ....  123 

Efficiency  of  a  Perfect  Engine 1 25 

Computations 126 

Calculating  Results ..'.....  127 

Duty  of  Pumps 131 


CONTENTS  ix 


CHAPTER  X 
THE  SLIDE  VALVE 

ARTICLE  PAGE 

Steam  and  Exhaust  Lap  .    .    .  & 133 

Valve  Without  Laps   7 134 

Valves  With  Lap. 136 

Position  of  Crank  and  Eccentric 137 

Lead , 138 

Angle  of  Advance 140 

Inside  Admission  Valve  .    .    : 141 

CHAPTER  XI 

THE  VALVE   DIAGRAM 

Valve  Displacement • 143 

Piston  Position 144 

Position  of  Crank  and  Eccentric 145 

Valve  Diagram 146 

CHAPTER   XII 

VALVE  SETTING 

General  Considerations 157 

Placing  an  Engine  on  Center 159 

To  Set  Valves  With  Equal  Leads 161 

Setting  Valves  for  Equal  Cut-off 162 

Types  of  Slide  Valves 164 

CHAPTER  XIII 

SHIFTING  ECCENTRIC  AND  MEYER  VALVE 

Shifting  Eccentric 171 

Effects  Produced  by  Slide  Valve 177 

Meyer  Valve 179 

CHAPTER    XIV 

REVERSING  MECHANISMS 

Reversing  Gears 183 

Stephenson  Link  Motion 184 

Watechaert  Valve  Gear 191 

Woolf  Reversing  Gear 196 

CHAPTER  XV 

CORLISS  VALVE  GEARS 

•     Advantages  of  the  Corliss  Valve ..'....    .    .    .   198 

Single  Eccentric  Valve  Gear.    .    .    . ;.'-.. -  .    •   200 

Setting  Corliss  Valves .    . 202 


CONTENTS 


CHAPTER  XVI 

GOVERNING 
ARTICLE  PAGE 

Governing 211 

Pendulum  Governor 212 

Stability    ............ 214 

Shaft  Governors .....' 220 

Inertia  Governor.    .    .    .  ^    , 222 

CHAPTER  XVII 
COMPOUND  ENGINES 

Compounding 225 

Expansion  of  Steam 227 

Compound  Engines 228 

Cross-compound  Engines 230 

Tandem-compound  Engines. 232 

Cross-compound  with  Receiver 233 

Power  of  a  Compound  Engine 235 

Advantages  and  Disadvantages .......  239 

CHAPTER  XVIII 
CONDENSING  APPARATUS 

Purpose  of  the  Condenser.    .    ...    ...    .    . .  _,  '. . 240 

Condensation  of  Steam t 242 

Measuring  Vacuum 243 

Forms  of  Condensing  Apparatus      246 

Jet  Condenser 247 

Siphon  Condensers. 248 

Barometric  Condenser 249 

Surface  Condensers 250 

High  Vacuum  Condensers 253 

Choice  of  a  Condenser 253 

CHAPTER  XIX 
LUBRICATION 

Friction i    ..........  255 

Lubrication  ....... 255 

Principles  of  Lubrication 256 

Characteristics  of  Oil '-...'. 259 

Testing  Oils <   .   .   .....  260 

Gumming  Test ,    .    .    .  260 

Flash  and  Fire  Tests 260. 

Acid  Test 261 

Steam-engine  Lubrication 261 


CONTENTS 

ARTICLE 

Lubricators  ..........................  ^ 

Lubrication  of  Valves.     Slide  Valve     .......  '  .......  266 

Corliss  Valves  ....    ,  .    .  ,.  "  ................  267 

Piston  Valves  .......  ..  ^    -        -.  •  .  ...........  2Q7 

Poppet  Valves.  ........  267 


Piston  and  Cylinders  ....'.. 
Piston  and  Valve  Rods 
Influence  of  Operating  Conditions 


CHAPTER  XX 
STEAM  TURBINES 

271 
General  Principles  ...................... 


STEAM*  ENGINES 


CHAPTER  I 
PRINCIPLES    OF   THE   STEAM    ENGINE 

Elementary  Principles. — In  the  steam  engine,  heat  energy  is 
changed  into  mechanical  energy.  The  pressure  of  steam  is  due 
to  the  heat  which  it  contains.  The  steam  pressure  acts  upon 
the  engine  piston,  causing  it  to  move,  and  thus  changes  the  heat 
energy  of  the  steam4nto  mechanical  energy.  The  kind  of  motion 
produced  is  a  backward  and  forward  motion  of  certain  parts  of 
the  engine.  This  kind  of  motion  is  called  a  reciprocating  motion 
and  the  parts  of  the  engine  which  have  this  kind  of  motion  are 
called  reciprocating  parts.  Other  parts  of  the  engine  change  the 
reciprocating  motion  into  a  rotary  motion  and  thus  the  engine 
may  turn  a  flywheel  continuously  and  transmit  the  motion  to 
other  machines. 

Figure  1  is  a  drawing  of  a  steam  engine,  simplified  in  order 
that  its  principles  may  be  more  readily  understood.  Practically 
all  steam  engines  operate  upon  the  same  principles,  hence  this 
explanation  will  serve  for  all  classes  of  engines.  Only  the  frame, 
cylinder,  piston,  piston  rod,  crosshead,  connecting  rod,  crank, 
shaft,  and  flywheel  are  shown  here.  The  frame  and  cylinder  are 
stationary  parts  and  the  others  are  moving  parts.  The  piston, 
piston  rod,  and  crosshead  have  a  reciprocating  motion,  and  the 
crank,  shaft,  and  flywheel  have  a  rotary  motion. 

The  piston  is  moved  backward  and  forward  by  the  pressure 
of  the  steam  which  is  admitted  first  to  one  end  of  the  cylinder  and 
then  to  the  other.  The  supply  of  steam  is  controlled  by  means  of 
a  valve  operated  by  the  engine  so  as  to  open  and  close  at  the 
proper  time.  This  valve,  which  is  an  important  part  of  the  engine 
mechanism,  is  not  shown  in  Fig.  1,  but  will  be  illustrated  and 
described  later. 

Referring  to  Fig.  1,  imagine  steam  to  be  admitted  to  the  right- 
hand  end  of  the  cylinder.  The  pressure  of  the  steam  acts  upon 

1 


2  STEAM  ENGINES 

the  piston  and  moves  it  to  the  left.  The  distance  which  the 
piston  travels  from  left  to  right  or  from  right  to  left  is  called  the 
stroke.  The  motion  of  the  piston  is  transmitted  through  the 
piston  rod  to  the  crosshead,  which  has  the  same  motion  as  the 
piston.  One  end  of  the  connecting  rod  is  connected  to  the  cross- 
head  and  moves  in  a  straight  line  with  it.  The  other  end  of  the 
connecting  rod  is  connected  to  the  crank  pin,  which  moves  in  a 
circle  about  the  center  of  the  shaft;  therefore,  the  connecting 
rod  changes  the  straight  line  motion  of  the  crosshead  into  the 
rotating  motion  of  the  shaft  and  flywheel. 


FIG.   1. 

When  the  steam  pressure  forces  the  piston  of  Fig.  1  to  the  left, 
the  crank  pin  rotates  through  the  top  -half  of  its  circular  path, 
moving  in  the  direction  of  the  arrow.  As  the  piston  moves  to  the 
left,  more  steam  enters  the  cylinder  and  maintains  a  constant 
pressure  upon  the  piston.  At  a  certain  point  in  the  stroke  of  the 
piston  the  valve  closes  and  stops  the  supply  of  steam  to  the  right- 
hand  end  of  the  cylinder.  This  point  in  the  stroke  is  called  the 
point  of  cut-off  or  simply  cut-off.  From  the  point  of  cut-off  to  the 
end  of  the  stroke  the  steam  behind  the  piston  expands  and  its 
pressure  diminishes.  At  the  end  of  the  stroke  from  right  to  left, 


PRINCIPLES  OF  THE  STEAM  ENGINE  3 

connection  between  the  right-hand  end  of  the  cylinder  and  the 
exhaust  pipe  is  opened  and  the  steam  begins  to  be  exhausted  from 
the  cylinder.  The  point  in  >ttie  stroke  at  which  steam  begins  to 
be  exhausted  from  the  cylinder  is  called  release. 

If  the  piston  should  stop  just  at  the  end  of  either  stroke,  the 
piston  rod,  connecting  rod,  and  crank  would  be  in  a  straight  line, 
and  the  engine  would  be  on  center  or  on  dead  center,  as  it  is  some- 
times called.  In  this  position  steam  pressure  acting  on  the  piston 
could  not  move  it,  since  the  pressure  would  be  simply  transferred 
to  the  bearings  of  the  shaft  and  there  would  be  no  turning  effect. 
However,  after  completing  a  stroke,  the  motion  of  the  parts  of  an 
engine  is  sufficient  to  carry  it  past  center  and  the  steam  pressure 
will  then  move  the  piston  forward. 

At  the  beginning  of  the  stroke  from  left  to  right,  the  valve 
admits  steam  to  the  left-hand  end  of  the  cylinder  while  keeping 
open  the  connection  between  the  right-hand  end  and  the  exhaust 
pipe.  The  steam  pressure  now  forces  the  piston  towards  the 
right,  and  the  crank  is  forced  through  the  bottom  half  of  its 
circular  path,  still  in  the  direction  of  the  arrow,  thus  causing  the 
shaft  and  flywheel  to  turn  continuously  in  the  same  direction. 
As  the  piston  moves  towards  the  right,  the  low  pressure  steam  in 
the  right-hand  end  of  the  cylinder  is  forced  out  by  the  piston. 
Steam  continues  to  be  admitted  to  the  left-hand  end  of  the 
cylinder  until  the  point  of  cut-off  in  this  stroke,  after  which  the 
steam  is  expanded  behind  the  moving  piston  until  the  end  of  the 
stroke,  when  exhaust  commences  from  the  left-hand  end.  Just 
before  the  piston  completes  its  stroke  from  left  to  right,  the 
connection  between  the  right-hand  end  of  the  cylinder  and  the 
exhaust  pipe  is  closed  and  the  steam  then  remaining  in  the  right- 
hand  end  of  the  cylinder  is  compressed  in  order  to  furnish  a 
cushion  for  the  returning  piston.  The  point  in  the  stroke  at 
which  the  exhaust  passage  is  closed  is  called  compression. 

When  an  engine  passes  through  a  regular  series  of  operations 
and  returns  at  regular  intervals  to  its  starting  point,  it  is  said  to 
perform  a  cycle.  The  parts  of  the  cycle  are  called  events.  The 
events  in  the  cycle  of  a  steam  engine  are  admission,  cut-off, 
release,  and  compression.  The  part  of  the  cycle  between  the 
point  of  admission  and  the  point  of  cut-off  .is  called  admission; 
the  part  between  the  point  of  cut-off  and  the  point  of  release  is 
called  expansion;  the  part  between  the  point  of  release  and  the 
point  of  compression  is  called  exhaust;  and  the  part  between  the 


4  STEAM  ENGINES 

point   of   compression   and   the   point   of  admission    is    called 
compression. 

The  series  of  operations,  admission,  expansion,  exhaust,  and 
compression  occurring  in  one  end  of  a  cylinder  make  up  the  cycle 
for  that  end.  If  the  cycle  is  performed  in  only  one  end  of  the 
cylinder  the  engine  is  said  to  be  single-acting,  but  if  the  cycle  is 
performed  in  each  end  of  the  cylinder,  as  in  the  engine  described 
above,  the  engine  is  said  to  be  double-acting.  Nearly  all  steam 
engines  are  double-acting,  since  a  double-acting  engine  has  about 
twice  the  power  of  a  single-acting  one  of  the  same  size.  In  a 
double-acting  engine  the  cycles  in  both  ends  of  the  cylinder  are 
being  performed  at  the  same  time,  admission  and  expansion  of 
one  cycle  occurring  in  one  end  of  the  cylinder  at  the  same  time 
that  exhaust  and  compression  of  the  other  cycle  are  occurring  in 
the  other  end  of  the  cylinder. 

The  end  of  the  cylinder  which  is  towards  the  shaft  or  flywheel 
is  called  the  crank  end,  and  the  opposite  end,  or  the  one  furthest 
from  the  shaft  or  flywheel,  is  called  the  head  end  of  the  cylinder. 
The  stroke  of  the  piston  from  the  head  end  of  the  cylinder  to  the 
crank  end  is  called  the  forward  stroke  and  the  stroke  from  the 
crank  end  to  the  head  end  is  called  the  return  stroke. 

When  the  piston  is  at  the  end  of  its  stroke  it  does  not  touch  the 
head  of  the  cylinder,  a  small  amount  of  clearance  between  them 
being  necessary.  The  space  between  the  head  of  the  cylinder 
and  the  piston  (when  it  is  at  the  end  of  its  stroke),  together  with 
the  volume  of  the  ports,  up  to  the  face  of  the  valves,  is  called 
the  clearance  volume,  or  simply  the  clearance.  The  clearance  is 
expressed  as  a  percentage  of  the  volume  displaced  by  the  piston 
during  a  single  stroke.  For  example,  the  clearance  of  an  engine 
may  be  12  per  cent.  If  a  20"  X  24"  engine  is  under  consideration 
(meaning  an  engine  having  a  cylinder  20  in.  in  diameter  with  a 
24  in.  stroke)  the  area  of  its  piston  is 

.7854  X  202  =  314.16  sq.  in. 
and  the  volume  displaced  during  a  single  stroke  is 
314.16  X  24  =  7539.84  cu.  in. 
7539.84  ,, 

°r  ~I728~    =  4'3°5  CU'  ft' 
The  clearance  volume  of  this  engine  is 

12  per  cent,  of  4.305  or 
.12  X  4.305  =  .5166  cu.  ft. 


PRINCIPLES  OF  THE  STEAM  ENGINE  5 

Parts  of  the  Steam  Engine. — The  different  parts  of  a  steam 
engine  in  their  relation  to  each,  other  are  shown  in  Fig.  2,  which 
represents  a  common  form  6f  steam  engine.  In  this  view  the 


cylinder  is  shown  cut  away  in  order  to  illustrate  its  interior 
construction. 

In  Fig.  2,  1  is  the  foundation  of  the  engine;  2  is  the  frame; 


6  STEAM  ENGINES 

3  is  the  cylinder;  4  is  the  head  end  cylinder  head;  5  is  the  crank 
end  cylinder  head;  6  is  the  piston;  30  is  the  piston  rod;  8  is  the 
crosshead;  9  and  9  are  the  crosshead  guides;  10  is  the  connecting 
rod;  12  and  12  are  the  cranks,  this  being  a  center  crank  engine; 
31  is  the  shaft;  13  is  the  flywheel;  14  is  the  eccentric;  15  is  the 
eccentric  strap;  16  is  the  eccentric  rod;  17  is  the  valve  stem  guide; 
18  is  the  valve  stem;  19  is  the  valve  which,  in  this  case,  is  a  slide 
valve;  20  and  21  are  the  steam  ports;  22  is  the  exhaust  port; 
23  is  the  steam  chest,  which  is  connected  to  the  steam  supply 
pipe;  24  is  the  exhaust  pipe,  which  is  connected  to  the  exhaust 
port;  25  is  the  piston  rod  stuffing  box;  26  is  the  valve  stem  stuffing 
box;  27  is  the  steam  chest  cover  plate;  and  28  is  the  lagging  or 
covering  for  the  cylinder. 

Classification  of  Engines. — Steam  engines  may  be  divided  into 
three  classes  depending  upon  their  type  of  valve  or  method  of 
controlling  the  speed,  and  these  classes  include  practically  all 
kinds  of  engines.     These  three  classes  are : 
The  plain  slide  valve  engine 
The  automatic  high  speed  engine 
The  Corliss  engine 

Any  of  the  above  types  may  be  classified  in  several  other  ways, 
among  which  are: 

According  to  the  position  of  the  cylinder,  as  horizontal  and 
vertical; 

According  to  the  number  of  cylinders  in  which  the  steam  is 
expanded,  as  simple,  compound,  triple  expansion,  and  quadruple 
expansion; 

According  to  the  manner  of  handling. the  exhaust  steam  as, 
condensing  and  noncondensing . 

A  horizontal  engine  is  one  whose  cylinder  is  placed  in  a  hori- 
zontal position  as  illustrated  in  Fig.  2,  while  a  vertical  engine 
is  one  having  the  cylinder  placed  vertically  and  directly  above  the 
shaft  as  shown  in  Fig.  3.  Some  of  the  largest  engines  are  a 
combination  of  horizontal  and  vertical,  having  one  horizontal 
and  one  vertical  cylinder,  and  the  connecting  rods  from  each 
of  these  connected  to  a  single  crank.  Phis  arrangement  is  used 
in  order  to  develop  a  large  amount  of  power  in  a  small  space. 
A  simple  engine  is  one  in  which  the  steam  is  expanded  in  only 
one  cylinder.  In  a  compound  engine  the  steam  is  first  expanded 
in  one  cylinder  and  the  exhaust  from  this  cylinder  is  led  to  a 
second  cylinder  where  it  is  expanded  further.  In  a  triple  expan- 


PRINCIPLES  OF  THE  STEAM  ENGINE 


7. 


sion  engine  the  total  expansion  of  the  steam  is  divided  into  three 
parts,  each  being  performed  in  a  separate  cylinder,  while  in  a 
quadruple  expansion  engine  tije  total  expansion  of  the  steam  is 
divided  into  four  parts,  each  being  performed  in  a  separate  cylin- 
der. The  general  names  of  multiple  expansion  or  compound  are 
used  to  designate  any  engine  in  which  the  expansion  is  performed 
in  more  than  one  cylinder.  The  reasons  for  dividing  the  expan- 


FIG.  3. 

sion  of  the  steam  into  parts  and  also  the  construction  of  multiple 
expansion  engines  will  be  taken  up  in  a  later  chapter. 

.A  condensing  engine  is  one  in  which  the  exhaust  steam  is 
changed  into  water.  Since  the  water  thus  formed  occupies 
less  space  than  the  exhaust  steam  the  back  pressure  against  which 
the  piston  must  make  its  return  stroke  is  reduced.  In  a  noncon- 
densing  engine  the  exhaust  steam  is  turned  into  the  atmosphere 
and  the  piston  must  return  against  the  pressure  of  the  atmosphere 
plus  enough  pressure  to  force  the  exhaust  steam  through  the 
exhaust  pipe  and  ports.  This  pressure  may  amount  to  from 


8 


STEAM  ENGINES 


18  to  20  Ibs.  per  sq.  in.  absolute,  or  3  to  5  Ibs.  per  sq.  in.  above 
atmospheric  pressure. 

The  Plain  Slide  Valve  Engine. — This  type  of  engine  is  named 
from  the  kind  of  valve  which  is  used  to  distribute  steam  to  the 


two  ends  of  its  cylinder,  this  type  of  valve  being  called  a  slide  valve. 
Figure  2  illustrates  a  common  form  of  plain  slide  valve  engine. 
The  slide  valve  mechanism  is  also  illustrated  in  Fig.  2  where  it  is 
shown  as  a  part  of  the  engine.  The  valve  and  mechanism  is 
again  shown  in  Fig.  4,  but  in  this  case  the  parts  are  placed  differ- 


PRINCIPLES  OF  THE  STEAM  ENGINE 


9 


ently  than  in  Fig.  2  iii  order  to  better  show  the  operation  of  the 
mechanism.  In  Fig.  4  the  valve  is  marked  1;  2  is  called  the 
valve  seat;  3  is  the  face  of  j}he  valve;  4  is  the  valve  rod;  5  is  the 
eccentric  rod;  6  is  the  eccentric;  7  is  the  shaft;  and  8  is  the 
eccentric  strap. 

The  slide  valve  moves  backward  and  forward  over  both  steam 
ports  and  the  exhaust  port,  and  is  thus  able  to  control  the  supply 
of  steam  to  both  ends  of  the  cylinder  and  also  the  exhaust  from 
both  ends.  The  valve  is  given  its  backward  and  forward  motion 
by  the  eccentric  which  is  fastened  by  set  screws  to  the  shaft. 
The  eccentric  consists  of  a  circular  disk  of  iron  having  its  center, 
O,  in  Fig.  4,  at  some  distance  from  the  center  of  the  shaft  N.  The 
center  of  the  eccentric  thus  moves  in  a  circle  about  the  center 


FIG.  5. 

of  the  shaft  and  in  this  way  has  a  motion  similar  to  that  of  a 
crank  with  a  length  NO.  A  crank  which  would  give  the  same 
motion  as  the  eccentric  is  shown  by  the  dotted  lines.  The  motion 
of  the  eccentric  is  transmitted  to  the  eccentric  rod  by  means  of 
a  strap,  8,  which  passes  around  the  eccentric.  The  eccentric 
rod  corresponds  to  a  connecting  rod  and  it  changes  the  circular 
motion  of  the  eccentric  into  a  reciprocating  motion,  which  is 
transmitted  to  the  valve  by  the  valve  rod. 

The  action  of  the  slide  valve  can  best  be  explained  by  consider- 
ing the  series  of  operations  which  occur  in  one  end  of  the  cylinder, 
remembering  that  similar  operations  are  occurring  in  the  other 
end  but  at  a  different  time.  In  Fig.  4  the  piston  is  at  the  head 
end  of  the  cylinder  and  just  beginning  its  forward  stroke,  the 
shaft  turning  in  the  direction  of  the  arrow. 

In  the  position  shown  in  Fig.  4,  the  valve  is  opening  to  admit 


10 


STEAM  ENGINES 


steam  to  the  head  end  of  the  cylinder.  The  steam  pressure  moves 
the  piston  towards  the  right  and  the  valve  is  opened  wider,  which 
allows  steam  to  flow  into  the  cylinder  more  freely.  The  valve 
soon  reaches  the  end  of  its  travel  towards  the  right  and  begins 
to  move  towards  the  left,  closing  the  head  end  steam  port.  Fig- 
ure 5  shows  the  valve  just  as  it  closes  the  head  end  steam  port 
which  cuts  off  the  supply  of  steam  to  the  cylinder.  It  will  be 
seen  from  this  figure  that  the  piston  has  still  some  distance  to 
go  before  completing  its  forward  stroke.  Figure  5  shows  the 
position  of  the  valve  and  piston  at  cut-off.  The  valve  continues 
to  move  towards  the  left,  keeping  the  steam  port  closed,  and  the 
steam  expands  behind  the  piston,  pushing  it  towards  the  right. 
By  the  time  the  piston  reaches  the  end  of  its  forward  stroke  the 
inner  edge  of  the  valve  begins  to  uncover  the  head  end  steam 


Fio.  6. 

port,  as  shown  in  Fig.  6,  and  gives  the  event  known  as  release. 
This  opens  communication  between  the  head  end  of  the  cylinder 
and  the  exhaust  port;  then  if  the  steam  still  has  any  pressure 
above  that  of  the  atmosphere,  this  pressure  immediately  drops 
to  the  exhaust  pressure. 

Steam  is  now  admitted  to  the  crank  end  of  the  cylinder  and  the 
piston  moves  towards  the  left,  pushing  the  spent  steam  in  the 
head  end  of  the  cylinder  into  the  exhaust  pipe.  This  part  of  the 
return  stroke  gives  exhaust  from  the  head  end  of  the  cylinder. 
The  valve  continues  to  move  towards  the  left,  opening  the  exhaust 
wider  and  wider,  until  it  reaches  the  end  of  its  travel,  when  it 
begins  to  move  towards  the  right  and  to  close  the  exhaust  port. 
When  the  piston  has  reached  the  point  in  its  return  stroke  shown 
in  Fig.  7,  th^  valve  has  moved  far  enough  to  the  right  to  close  the 


PRINCIPLES  OF  THE  STEAM  ENGINE 


11 


exhaust  port.  From  this  point  to  the  end  of  the  return  stroke, 
-the  exhaust  passage  remains  closed,  and  the  piston  compresses 
the  steam  which  remains  in  ,tKe  head  end  of  the  cylinder  so  that 
at  the  end  of  the  stroke  the  clearance  volume  of  the  cylinder  is 
filled  with  high  pressure  steam.  This  completes  the  cycle  in 
the  head  end  of  the  cylinder. 

By  referring  to  Figs.  4,  5,  6,  and  7  it  will  be  seen  that  the 
valve  is  so  constructed  that  at  the  same  time  admission  and 
expansion  are  occurring  in  the  head  end  of  the  cylinder,  exhaust 
and  compression  are  occurring  in  the  crank  end ;  and  at  the  same 
time  that  exhaust  and  compression  are  occurring  in  the  head  end, 
admission  and  expansion  are  occurring  in  the  crank  end.  Thus 
the  cycles  for  both  ends  of  the  cylinder  are  performed  in  their 


FIG.  7. 


proper  order  and  the  engine  made  to  run  continuously  by  means 
of  a  single  slide  valve  and  a  single  eccentric. 

Speed  Regulation. — The  speed  of  a  steam  engine  may  vary  in 
two  ways  and  from  two  different  causes.  First,  there  may  be  a 
variation  during  each  stroke  on  account  of  the  changing  steam 
pressure  in  the  cylinder.  In  the  first  part  of  the  stroke  when  the 
steam  pressure  is  high  there  is  a  tendency  for  the  speed  to  increase 
and  in  the  last  part  of  the  stroke  when  the  steam  pressure  has 
been  reduced  by  expansion  there  is  a  tendency  for  the  speed  to  be 
lower.  This  variation  of  speed  is  entirely  independent  of  the  load 
carried  by  the  engine  and  it  would  occur  whether  the  load  upon 
the  engine  was  large  or  small. 

The  variation  of  speed  during  a  single  stroke  is  counteracted 
by  the  action  of  the  flywheel.  It  is  a  well-known  fact  that  when  a 


12  STEAM  ENGINES 

heavy  object  is  moving  it  is  difficult  to  change  its  speed.  This 
property  of  a  body  is  called  its  inertia.  The  inertia  of  the  flywheel 
is  used  to  counteract  the  changes  of  speed  during  a  single  stroke. 
The  flywheel  absorbs  energy  during  the  first  part  of  the  stroke 
when  the  speed  tends  to  increase  and  gives  it  out  again  during 
the  last  part  of  the  stroke  when  the  speed  tends  to  decrease. 
A  heavy  flywheel  will  keep  the  speed  of  an  engine  steadier  than  a 
light  one  and  weight  concentrated  at  the  rim  is  more  effective 
than  weight  nearer  the  hub,  hence  flywheels  which  are  intended 
to  steady  the  speed  are  usually  made  with  a  very  heavy  rim. 

Besides  the  variation  of  speed  mentioned  above,  there  is  also 
a  variation  due  to  changes  in  the  load  which  the  engine  carries. 
Changes  in  the  load  affect  the  speed  for  a  longer  period  than  a 
single  stroke  and  they  cannot  be  controlled  by  the  action  of  the 
flywheel.  If  the  amount  of  steam  supplied  to  the  cylinder  at 
each  stroke  is  constant  and  the  load  increases,  the  speed  of  the 
engine  will  decrease  until  the  power  developed  in  the  cylinder 
balances  the  load  on  the  engine,  and  if  the  load  decreases  the 
speed  increases  until  the  power  developed  in  the  cylinder  again 
balances  the  load.  The  load  on  most  engines  is  varying  all  the 
time  and,  as  it  is  desirable  to  keep  the  speed  constant,  some  means 
must  be  provided  for  varying  the  amount  of  steam  supplied  to 
the  engine  according  to  the  load  it  is  carrying,  so  that  when  the 
load  increases  the  amount  of  steam  supplied  will  be  greater  and 
when  the  load  decreases  the  amount  of  steam  supplied  will  be 
less.  This  is  called  governing  the  engine,  and  the  mechanism  for 
controlling  the  steam  supply  is  called  a  governor. 

The  governor  of  a  steam  engine  may  control  the  speed  by 
changing  the  steam  pressure  or  by  changing  the  volume  of  steam 
admitted  to  the  cylinder  during  the  period  of  admission.  In  the 
first  method  the  governor  operates  a  throttle  valve  placed  in  the 
main  steam  pipe  where  it  enters  the  steam  chest  and  the  partial 
closing  of  this  valve  reduces  the  steam  pressure.  By  this  means 
the  governor  controls  the  steam  pressure  acting  upon  the  piston 
to  agree  with  the  load  on  the  engine.  This  kind  of  governor  is 
called  a  throttling  governor. 

In  the  second  method  mentioned  above  the  governor  is 
arranged  so  as  to  change  the  point  of  cut-off  and  thus  change  the 
volume  of  steam  admitted  to  the  cylinder  to  agree  with  the  load. 
When  the  load  increases  the  governor  makes  the  point  of  cut-off 
occur  later  in  the  stroke  thus  admitting  more  steam  to  the 


PRINCIPLES  OF  THE  STEAM  ENGINE  13 

cylinder,  and  when  the  load  decreases  the  point  of  cut-off  occurs 
earlier,  admitting  a  smaller  volume  of  steam  to  the  cylinder. 

An  engine  whose  speed  is.  regulated  by  throttling  or  reducing 
the  pressure  of  the  steam  supply  uses  a  large  amount  of  steam  in 
proportion  to  the  work  it  performs,  or  is  inefficient,  because  the 
full  pressure  of  the  steam  is  used  only  when  the  load  is  greatest 
and  for  any  smaller  load  a  portion  of  the  steam  pressure  is  wasted. 
The  method  of  governing  in  which  the  volume  of  steam  admitted 
to  the  cylinder  is  changed  is  more  economical  because  all  of  the 
steam  admitted  to  the  cylinder  is  used  at  the  full  boiler  pressure. 

In  the  plain  slide  valve  engine  the  position  of  the  eccentric 
determines  the  part  of  the  stroke  at  which  cut-off  occurs.  Since 
the  eccentric  is  fastened  to  the  shaft  the  point  of  cut-off  occurs  at 
a  fixed  point  in  the  stroke,  therefore  the  speed  is  governed  by  the 
throttling  method. 

The  plain  slide  valve  engine  is  usually  designed  to  run  at 
slow  or  medium  speeds,  with  a  stroke  somewhat  greater  than  the 
diameter  of  the  piston.  Most  of  them  are  of  small  size,  since 
they  are  uneconomical  in  the  use  of  steam.  They  are  simple  in 
construction  and  cheap  in  cost,  hence  are  much  used  where  only 
a  small  amount  of  power  is  needed  and  where  expert  attendance 
is  not  obtainable.  With  ordinary  care,  they  last  a  long  time  and 
do  not  easily  get  out  of  order.  This  type  of  engine  uses  from  35 
to  60  pounds  of  steam  per  hour  for  each  horsepower  developed. 

Automatic  High  Speed  Engines. — The  type  of  engine  known 
as  the  automatic  high  speed  engine  is  also  a  slide  valve  engine, 
but  its  valve  differs  somewhat  from  that  used  in  the  plain  slide 
valve  engine  and  it  also  differs  in  other  details  of  construction. 

The  valves  used  on  the  automatic  high  speed  type  of  engine  are 
of  much  better  construction  than  those  used  on  the  plain  slide 
valve  engine.  One  kind  of  valve  commonly  used  on  these  engines 
is  illustrated  in  Fig.  8.  This  kind  of  valve  is  known  as  a  balanced 
valve  because  it  has  a  balance  plate  which  prevents  the  steam 
pressure  from  acting  on  the  back  of  the  valve.  The  plain  slide 
valve,  which  has  no  balance  plate,  has  the  steam  pressure  in  the 
steam  chest  acting  on  the  entire  area  of  the  back  of  the  valve. 
As  the  area  of  the  valve  is  large,  the  valve  is  pressed  against  its 
seat  with  an  enormous  pressure,"  requiring  a  large  amount  of 
work  in  moving  the  valve.  The  large  area  of  the  valve  also 
makes  it  difficult  for  oil  to  get  between  the  valve  and  its  seat  to 
lubricate  it.  Some  valves  have  balance  plates  which  cover  about 


14 


STEAM  ENGINES 


80  per  cent,  of  the  area  of  the  valve,  leaving  20  per  cent,  of  its 
area  upon  which  the  steam  pressure  may  act.  This  part  of  the 
steam  pressure  is  sufficient  to  keep  the  valve  properly  seated 


FIG.  8. 


and  is  not  enough  to  cause  undue  friction.  The  balance  plate 
is  adjustable  to  allow  for  wear  of  the  valve,  being  held  in  position 
by  screws  which  press  against  the  sides  of  the  steam  chest  and 
hold  it  in  place. 


FIG.  9. 

Another  kind  of  valve,  commonly  used  on  automatic  high 
speed  engines,  is  illustrated  in  Fig.  9.  This  form  of  valve  is 
made  in  the  shape  of  a  spool  and  is  like  a  slide  valve  which  has 
been  curved  into  a  cylindrical  form.  Its  motion  is  the  same  as 
that  of  the  plain  slide  valve.  In  the  valve  shown  here,  which  is 


PRINCIPLES  OF  THE  STEAM  ENGINE 


15 


called  a  piston  valve,  steam  enters  the  steam  chest  at  the  central 
part  of  the  valve  and  is  admitted  to  the  cylinder  past  the  inner 
edges  of  the  valve,  exhaust  jtaking  place  past  the  outer  edges. 
This  is  the  reverse  of  the  mangier  in  which  a  plain  slide  valve 
admits  and  exhausts  steam.  The  steam  ports  completely  sur- 
round the  valve  so  that  a  large  port  opening  is  secured  with  a 
small  movement  of  the  valve.  This  valve  does  not  require  a 
balancing  plate  because  the  steam  pressure  acts  on  all  sides 
of  the  valve  equally,  thus  making  it  perfectly  balanced. 

In  the  plain  slide  valve  engine  the  volume  of  steam  admitted 
to  the  cylinder  at  each  stroke  of  the  piston  is  constant,  the  speed 
of  the  engine  being  controlled  by  changing  the  admission  pressure 


FIG.  10. 

of  the  steam.  In  the  automatic  high  speed  engine,  the  admission 
pressure  of  the  steam  is  constant,  the  speed  being  controlled  by 
changing  the  point  of  cut-off,  and  thus  regulating  the  volume  of 
steam  admitted  to  the  cylinder  to  suit  the  amount  of  work  being 
done.  The  latter  is  the  more  economical  method  of  controlling 
the  speed  because  the  full  steam  pressure  is  utilized  all  the  time. 
The  point  of  cut-off  is  changed  in  the  high  speed  engine  by  means 
of  a  governor  which  is  attached  to  the  eccentric  in  such  manner 
that  the  eccentric  may  be  shifted  around  on  the  shaft.  The 
mechanism  for  doing  this  will  be  fully  described  in  the  chapter 
on  governors. 

Figure  10  is  a  side  view  of  an  automatic  high  speed  engine,  and 
serves  to  show  the  proportion  of  its  parts.  It  will  be  noticed 
that  the  engine  is  self-contained,  that  is,  it  rests  on  a  bedplate 


16  STEAM  ENGINES 

which  forms  a  foundation  for  it.  These  engines  are  usually 
short,  the  parts  being  grouped  closely.  As  compared  with  a 
plain  slide  valve  engine,  the  automatic  high  speed  engine  has  a 
shorter  cylinder  and  connecting  rod  in  proportion  to  the  diameter 
of  the  cylinder.  It  is  made  in  these  proportions  because  a  piston 
speed  of  about  500  to  700  feet  per  minute  is  desirable  for  all 
classes  of  engines  and  in  order  to  secure  this  piston  speed  the 
length  of  stroke  must  be  short  if  the  number  of  revolutions  per 
minute  is  large. 

The  automatic  high  speed  engine  is  made  for  speeds  up  to 
about  350  revolutions  per  minute  and  in  size  up  to  about  600 
horsepower.  It  has  a  close  speed  regulation  at  all  loads  and  is, 
therefore,  well  adapted  for  direct  connection  to  electric  genera- 
tors, a  class  of  work  which  requires  high  speeds  and 
close  speed  regulation.  These  engines  are  also  often  con- 
nected to  line  shafting  by  means  of  belts,  and  used  for  general 
power  purposes.  They  are  more  efficient  than  the  plain  slide 
valve  engine,  using  from  30  to  40  pounds  of  steam  per  hour  per 
horsepower. 


CHAPTER  II 
CORLISS  AND  OTHER  ENGINES 

Corliss  Engines. — The  Corliss  engine  is  an  entirely  different 
type  from  either  the  plain  slide  valve  or  the  automatic  high  speed 
type.  This  type  of  engine,  like  the  others,  is  named  from  its 
type  of  valve,  which  is  known  as  the  Corliss  valve. 

The  Corliss  valve  is  cylindrical  in  shape  and  is  placed  across 

ft  ff  ff  ft 


FIG.  11. 

the  cylinder  instead  of  parallel  with  it.  There  are  four  of  these 
valves  for  each  cylinder,  one  admission  valve  for  each  end  of  the 
cylinder  and  one  exhaust  valve  for  each  end.  Each  of  these 
valves  rotates  about  its  axis  instead  of  moving  backward  and 


VALVE   STEM 


WOPKINO  EDGE 
FIG.  12. 


forward  parallel  with  its  axis,  as  does  the  piston  type  of  slide 
valve.  The  Corliss  valve  does  not  turn  through  a  complete 
revolution,  but  oscillates  back  and  forth  through  an  angle  only 
large  enough  to  uncover  the  port. 

A  cross  section  of  a  cylinder  fitted  with  Corliss  valves  is  shown 

17 


18 


STEAM  ENGINES 


in  Fig.  11,  and  a  view  of  one  of  the  valves  removed  from  the 
cylinder  is  shown  in  Fig.  12.  It  will  be  observed  that,  by  this 
arrangement  of  valves,  the  ports  are  made  short  and  the  clear- 
ance reduced.  The  ports  extend  across  the  cylinder  and  are 
about  equal  in  length  to  the  diameter  of  the  cylinder.  Steam 
pressure  acts  upon  the  backs  of  the  valves  keeping  them  pressed 
against  their  seats,  but  the  friction  is  small  since  the  valves  are 
small  and  they  travel  only  a  short  distance  in  rotating  through 
a  small  angle.  The  travel  of  the  valve,  in  order  to  gain  a  full 
port  opening,  is  sometimes  further  decreased  by  having  two  open- 
ings through  the  valve,  or  making  them  " double  ported"  instead 
of  having  the  steam  pass  only  one  edge  of  the  valve.  By  having 


FIG.  13. 

two  ports  through  each  valve,  the  valve  need  travel  only  one- 
half  as  far  for  a  given  port  opening  as  would  a  single  ported 
valve. 

The  Corliss  valve  is  operated  from  an  eccentric,  the  same  as 
a  slide  valve,  but  it  differs  from  the  slide  valve  in  that  the  admis- 
sion valves  are  connected  to  the  eccentric  only  at  those  times 
when  they  are  being  opened.  The  exhaust  valves  are  connected 
to  the  eccentric  at  all  times,  but  the  mechanism  for  moving  them 
is  such  that  they  move  through  a  very  small  angle,  thus  reducing 
friction. 

The  mechanism  by  which  the  Corliss  valve  is  operated  is  shown 
in  Fig.  13  in  which  some  of  the  rods  are  represented  by  single 
lines  in  order  to  make  the  drawing  clearer.  As  shown  in  this 


CORLISS  AND  OTHER  ENGINES 


19 


drawing,  the  admission  valves  are  at  the  top  of  the  cylinder  and 
the  exhaust  valves  at  the  bottom. 

The  eccentric  rod  is  connected  at  F  to  the  rocker  arm  £,  which 
is  pivoted  at  D,  so  that  the  enctyG,  of  the  rocker  arm  swings  back- 
and  forth  through  a  small  angle.  A  wheel,  4,  called  a  wrist 
plate,  which  is  free  to  turn,  is  placed  on  the  side  of  the  cylinder, 
and  a  point,  H,  on  its  rim  is  connected  by  a  rod  to  the  upper  end, 
G,  of  the  rocker  arm.  The  wrist  plate,  therefore,  oscillates 
back  and  forth  through  the  same  angle  as  the  upper  end  of  the 
rocker  arm.  All  of  the  valves  are  connected  to  the  wrist  plate 
by  means  of  the  rods  5,  5,  and  7,  7,  and  take  their  motion  from  it. 

The  exhaust  valves  are  fitted  with  round  stems  which  pass 
through  stuffing  boxes  in  the  side  of  the  cylinder,  so  they  are 


FIG.   14. 

free  to  turn  without  allowing  steam  to  leak  past  them.  To  the 
ends  of  the  exhaust  valve  stems  are  fitted  cranks,  8  and  8,  which 
are  connected  by  rods,  7  and  7,  to  the  rim  of  the  wrist  plate. 
The  oscillation  of  the  wrist  plate  causes  these  cranks  to  move 
back  and  forth  through  a  small  angle,  thus  opening  and  closing 
the  exhaust  valves. 

The  admission  valves  are  not  connected  directly  to  the  wrist 
plate,  but  are  arranged  so  that  they  are  connected  to  it  while  the 
valves  are  being  opened,  and  disconnected  while  the  valves  are 
being  closed.  The  device  by  which  this  is  accomplished  is  shown 
in  detail  in  Fig.  14.  In  this  figure,  the  round  valve  stem  project- 
ing through  its  stuffing  box  is  shown  at  9  in  the  figure  at  the  left. 
The  end  of  the  valve  stem  is  supported  in  a  yoke,  B,  called  a 
bonnet,  which  is  bolted  to  the  side  of  the  cylinder.  The  end  of 
the  bonnet  is  finished  round  and  upon  it  are  fitted  two  cranks, 


20  STEAM  ENGINES 

6  and  11,  which  are  free  to  turn.  The  crank  6  is  L-shaped  and  is 
called  a  bell  crank.  One  arm  of  the  bell  crank  is  connected  by  a 
rod,  5,  to  the  wrist  plate;  the  other  end  carries  a  V-shaped  hook, 
10,  called  a  steam-hook,  which  is  pivoted  at  J.  The  end  of  the 
valve  stem  carries  a  crank,  9,  which  is  keyed  to  it  beyond  the  end 
of  the  bonnet.  The  end  of  this  crank  has  upon  it  a  small  square 
steel  block,  P,  and  there  is  another  small  steel  block,  L,  upon  the 
end  of  the  hook-claw.  As  the  wrist  plate  oscillates  back  and 
forth  the  bell  crank,  6,  moves  over  far  enough  so  its  steel  block,  L, 
hooks  under  the  steel  block,  P,  on  the  valve  stem  crank.  As 
the  bell  crank  moves  back,  the  valve  stem  crank  is  pulled  with  it, 
thus  opening  the  valve.  At  a  certain  point  in  the  backward 
movement  of  the  bell  crank  the  governor  causes  the  steam-hook, 
10,  to  release  the  valve  stem  crank  and  the  valve  is  closed,  which 
causes  steam  to  be  cut  off  from  that  end  of  the  cylinder.  The 
crank,  11,  is  carried  by  the  bonnet  and  is  free  to  turn  upon  it. 
This  crank  is  connected  to  the  governor  by  the  rod  12.  The 
crank  11  has  a  small  steel  block,  d,  fastened  on  its  hub,  and  the 
position  of  this  steel  block  is  determined  by  the  position  of 
the  governor  which  is  connected  to  the  crank  11.  As  the  V-- 
shaped hook-claw  moves  backward  after  picking  up  the  valve 
stem  crank,  the  end,  jP,  of  the  hook-claw  strikes  the  steel  block 
on  crank  11  and  causes  the  hook-claw  to  release  the  valve  stem 
crank.  Referring  to  Fig.  13,  it  will  be  seen  that  the  wrist  plate 
is  near  the  extreme  of  its  travel  towards  the  right  and  that  the 
hook-claw  on  the  right-hand  end  is  just  ready  to  pick  up  the 
valve  stem  crank.  At  the  same  time  the  valve  on  the  left-hand 
end  has  been  opened  and  is  ready  to  be  released  by  the  governor. 

The  admission  valves  are  made  to  close  quickly  by  means  of  a 
dashpot,  which  is  shown  at  15  in  Fig.  13.  The  dashpot  consists 
of  a  cylinder  16  and  a  piston  P2.  The  piston  is  connected  by  the 
rod  14  to  the  valve  stem  crank  so  that  as  this  crank  is  raised  upon 
opening  the  valve,  the  piston  in  the  dashpot  is  also  raised.  Rais- 
ing the  dashpot  piston  causes  a  vacuum  in  the  dashpot  which 
exerts  a  powerful  suction  on  the  piston  and  closes  the  valve 
quickly  when  it  is  released,  thus  cutting  off  the  supply  of  steam 
suddenly. 

It  will  be  seen  from  the  above  description  of  the  valve  mechan- 
ism that  the  method  of  governing  the  speed  of  the  Corliss  engine 
is  by  changing  the  volume  of  steam  admitted  to  the  cylinder  at 
each  stroke  which  is  the  most  economical  method  of  governing. 


CORLISS  AND  OTHER  ENGINES  21 

The  automatic  high  speed  engine  uses  this  method  of  governing, 
also;  but  in  this  case  the  arrangement  of  the  valve  mechanism  is 
such  that  when  the  point  of » cut-off  changes,  the  point  of  com- 
pression changes  also,  as  will  be  seen  after  this  type  of  valve 
mechanism  is  studied  in  a  later  chapter.  In  the  automatic 
engine  the  valve  closes  somewhat  gradually  which  causes  the 
admission  pressure  to  decrease  gradually  up  to  the  point  of 
cut-off,  while  in  the  Corliss  engine  the  valve  opens  wide  at 
admission,  remains  wide  open  during  admission,  and  closes 
suddenly  at  cut-off,  thus  giving  full  steam  pressure  upon  the 
piston  during  the  entire  admission.  At  light  loads  the  valve 
does  not  open  full  port  area,  but  it  generally  opens  enough  to 
give  full  initial  pressure.  Also,  since  the  steam  valves  are  inde- 
pendent of  the  exhaust  valves,  the  point  of  compression  does  not 
change  when  the  point  of  cut-off  changes  in  regulating  the  speed. 
On  some  Corliss  engines  the  exhaust  valves  are  not  only  inde- 
pendent of  the  steam  valves  but  they  are  operated  from  a  separate 
eccentric.  The  reasons  for  this  will  be  explained  fully  in  a 
later  chapter. 

There  are  many  modifications  of  the  Corliss  valve  mechanism, 
but  the  principle  is  the  same  in  all  of  them,  that  is,  the  valve  is 
opened  quickly  by  the  eccentric,  is  disengaged  from  the  eccentric 
at  the  proper  time  by  the  governor,  and  closed  quickly  by  a 
vacuum  dashpot. 

The  valve  mechanism  is  the  distinctive  feature  of  the  Corliss 
engine,  as  aside  from  it  this  engine  differs  but  little  from  any 
other  type  of  slow  speed  engine.  The  invention  of  the  Corliss 
valve  mechanism  is  the  greatest  development  that  has  taken  place 
since  the  invention  of  the  present  form  of  steam  engine  by  James 
Watt  in  1769.  The  high  efficiency  of  the  Corliss  engine  is  due 
to  its  form  of  valve  mechanism  more  than  to  anything  else  as  it 
permits  an  early  cut-off  with  large  expansion  of  the  steam  and 
does  not  throttle  the  steam  pressure  during  admission  or  exhaust. 

A  general  view  of  a  Corliss  engine  is  shown  in  Fig.  15,  which 
serves  to  give  an  idea  of  the  proportion  of  its  parts.  It  will  be 
observed  that  this  type  of  engine  has  a  somewhat  longer  cylinder 
in  proportion  to  its  diameter  and  also  a  longer  connecting  rod 
than  other  types  of  engines.  This  gives  the  whole  engine  an 
appearance  of  considerable  length  in  proportion  to  its  height. 

The  complicated  valve  mechanism  used  on  the  Corliss  engines 
makes  it  necessary  to  run  them  at  a  relatively  low  speed  in  order 


22 


STEAM  ENGINES 


CORLISS  AND  OTHER  ENGINES  23 

for  the  various  parts  to  adjust  themselves  and  for  the  dash  pot 
piston  to  work  properly.  In  order  to  maintain  a  proper  piston 
speed  with  the  slow  rate  of  revolution,  the  cylinder  is  made  long 
in  proportion  to  its  diameter.  Corliss  engines  are  rarely  made  to 
run  at  a  higher  speed  than  100  to  125  revolutions  per  minute,  and 
in  the  larger  sizes  the  speed  is  even  slower  than  this. 

The  Corliss  engine  is  the  most  efficient  type  of  steam  engine. 
It  rarely  uses  over  25  pounds  of  steam  per  hour  per  horsepower 
and  in  the  larger  sizes  its  steam  consumption  is  much  less  than 
this.  This  type  of  engine  is  made  in  sizes  from  100  to  12,000 
horse  power.  The  Corliss  engine  is  particularly  adapted  to 
running  mills  and  for  other  power  purposes  on  account  of  its 
smooth  running  qualities  and  its  close  speed  regulation.  The 
speed  of  the  Corliss  engine  is  controlled  by  changing  the  point 
of  cut-off  to  suit  the  load,  thus  controlling  the  volume  of  steam 
admitted  to  the  cylinder,  the  admission  pressure  of  the  steam 
remaining  constant. 

Besides  the  three  main  types  of  engines  mentioned  above  there 
are  various  modifications  of  these  types  which  are  used  extensively 
and  which  are  important  on  this  account.  Some  of  the  principal 
ones  of  these  are  described  below. 

Nonreleasing  Corliss  Engine. — The  nonreleasing  Corliss  engine 
is  a  type  of  medium  and  high  speed  engine  which  has  been  devel- 
oped in  recent  years.  It  is  a  combination  of  the  automatic  high 
speed  and  the  Corliss  engines  and  it  is  used  in  the  same  kind  of 
service  as  the  automatic  engine,  that  is,  for  direct  connection  of 
electric  generators  and  for  general  power  purposes  by  belting  to 
line  shafting.  On  account  of  its  high  speed  this  engine  has  the 
general  shape  and  proportions  of  the  automatic  high  speed  engine, 
as  will  be  seen  from  Fig.  16,  which  shows  one  of  these  engines. 

Any  engine  which  runs  at  a  high  speed  must  have  a  positive 
connection  between  the  eccentric  and  valve  because  there  would 
not  be  time  for  a  disengaging  mechanism  to  operate  properly. 
For  this  reason  the  nonreleasing  Corliss  engine  has  its  admission 
valves,  which  are  of  the  Corliss  type,  connected  directly  to  a 
reach  rod  which  is  operated  by  an  eccentric  rod  from  the  eccen- 
tric connected  to  the  governor.  With  the  Corliss  type  of  valve 
connected  directly  to  the  governor  eccentric,  the  nonreleasing 
Corliss  engine  has  some  of  the  characteristics  of  the  Corliss 
engine  and  some  of  the  automatic  high  speed  engine. 

The  method  of  governing  the  speed  of  the  nonreleasing  Corliss 


24  STEAM  ENGINES 

engine  is  the  same  as  that  used  with  the  automatic  high  speed 
engine,  that  is,  the  point  of  cut-off  is  changed  by  shifting  the 
eccentric  which  operates  the  admission  valves  around  on  the 
shaft.  This  does  not  change  the  point  of  compression  because 
the  exhaust  valves  are  operated  by  a  separate  eccentric  which  is 
fastened  to  the  shaft.  This  gives  a  steam  distribution  to  the 
cylinder  even  better  than  that  given  by  the  automatic  high  speed 
engine  with  its  slide  valve,  because  release  and  compression 
can  be  fixed  at  the  most  advantageous  points  and  they  remain 
unchanged  when  the  cut-off  is  changed.  A  double  ported  valve, 
which  reduces  the  necessary  movement  of  the  valve  for  the  same 
port  opening  and  also  further  reduces  the  friction,  is  used  in  some 
of  these  engines.  The  type  of  valve  used  on  this  engine  permits 


FIG.  16. 

short  ports  which  reduce  the  clearance  volume  and  thereby 
increase  the  economy  of  the  engine.  However,  at  least  6  per  cent, 
clearance  is  needed  for  quiet  running. 

In  a  slide  valve  engine  the  same  valve  is  used  both  for  admission 
and  exhaust.  The  exhaust  steam,  having  a  lower  temperature 
than^  the  admission  steam,  chills  the  valve  so  that  when  the  next 
admission  occurs,  a  part  of  the  admission  steam  is  condensed  by 
coming  in  contact  with  the  cooler  valve.  This  condensation, 
though  small  is  avoided  in  the  nonreleasing  Corliss  engines  since 
there  are  separate  admission  and  exhaust  valves. 

Since  the  non-releasing  Corliss  engine  combines  some  of  the 
advantages  of  the  Corliss  engine  with  those  of  the  automatic 
high  speed  engine,  its  steam  consumption  per  horsepower  per 
hour  is  slightly  greater  than  that  of  the  Corliss  engine  but  less 


CORLISS  AND  OTHER  ENGINES  25 

than  that  of  the  automatic  high  speed  engine.  It  is  made  in 
sizes  up  to  about  600  horsepower  and  for  speeds  up  to  about 
350  revolutions  per  minute.  > 

The  Locomotive. — The  locoinotive,  familiar  to  everyone,*  is  a 
type  of  plain  slide  valve  engine,  differing  from  the  ordinary  plain 
slide  valve  engine  in  having  the  valve  mechanism  arranged  so 
that  the  direction  of  rotation  may  be  reversed  at  will.  This  type 
of  valve  mechanism  will  be  studied  in  detail  in  a  later  chapter. 
Instead  of  having  a  single  engine  the  locomotive  has  two  complete 
engines,  one  on  each  side  connected  to  the  main  driving  shaft 
by  cranks  placed  90°  apart  so  the  locomotive  may  be  started 
from  any  position  even  if  one  engine  is  on  dead  center. 

The  valves  used  on  locomotive  engines  are  of  the  balanced 
type,  some  of  them  being  flat  valves  with  balance  rings  or  plates 
on  the  back,  and  some  being  cylindrical  or  piston  valves  which  are 
completely  balanced.  In  both  cases  the  usual  location  of  the 
valve  is  on  top  of  the  cylinder  where  it  is  accessible  to  the 
engineer. 

Unlike  the  engines  perviously  mentioned,  the  locomotive  has 
no  flywheel,  its  function  being  performed  by  the  driving  wheels 
and  by  the  weight  of  the  boiler  resting  on  them.  Neither  has  the 
locomotive  engine  a  governor  since  it  is  not  intended  to  run  at 
constant  speed.  Its  speed  is  controlled  by  hand  to  suit  the  load 
which  the  engine  is  pulling.  There  are  two  means  of  regulating 
the  speed :  first  by  means  of  a  hand-operated  throttle  valve  which 
controls  the  pressure  of  the  steam  admitted  to  the  cylinders;' 
and  second,  by  changing  the  point  of  cut-off,  which  may  be  done 
by  means  of  the  reversing  mechanism. 

Until  recent  years  the  locomotive  consisted  of  two  simple 
engines.  The  demand  for  greater  power  has  brought  about  the 
development  of  the  compound  locomotive  in  which  the  steam  is 
first  expanded  in  a  high  pressure  cylinder  and  then  in  a  low 
pressure  cylinder,  just  as  in  the  stationary  compound  engine. 
In  some  locomotives  the  high  pressure  cylinder  is  on  one  side  and 
the  low  pressure  cylinder  on  the  other.  This  arrangement  re- 
quires some  provision  whereby  high  pressure  steam  may  be  ad- 
mitted to  the  low  pressure  cylinder  in  order  to  start  when  the  high 
pressure  side  is  on  center.  Another  common  arrangement  of 
the  cylinders  is  to  place  one  high  and  one  low  pressure  cylinder, 
on  each  side  of  the  locomotive,  thus  making  the  locomotive 
consist  of  two  complete  compound  engines.  In  this  arrangement 


26  STEAM  ENGINES 

of  cylinders,  the  high  pressure  cylinder  is  placed  directly  above 
and  parallel  to  its  low  pressure  cylinder,  the  piston  rods  from  both 
cylinders  connecting  to  a  single  crosshead.  This  gives  a  more 
compact  and  powerful  engine  than  the  other  arrangement  of 
compound  cylinders. 

A  locomotive  is  a  complete  power  plant  in  itself  consisting  of 
both  boiler  and  engine,  and,  in  some  cases,  of  a  feed  water  heater 
and  superheater  also.  The  amount  of  power  developed  is  large' 
compared  with  the  size  of  the  boiler  and  engine,  being  as  much  as 
2000  Hp.  in  some  cases.  When  it  is  considered  that  this  power 
is  sometimes  developed  with  a  steam  consumption  of  about  20-24 
pounds  of  steam  per  horsepower  per  hour,  the  efficiency  of  these 
machines  is  wonderful. 

Marine  Engines.- — Engines  used  on  steamships  form  another 
distinct  class  of  steam  engines.  These  are  also  of  the  plain  slide 
valve  type  and,  like  the  locomotive,  are  provided  with  a  mechan- 
ism for  reversing  the  direction  of  rotation  of  the  engine.  Marine 
engines  are  vertical  and  are  invariably  multiple  expansion  in 
order  to  secure  a  large  amount  of  power  within  a  small  space. 
The  use  of  several  cranks  also  gives  a  more  uniform  turning  effort 
to  the  shaft.  The  engines  are  compound,  triple  or  quadruple 
expansion  depending  largely  upon  the  size  of  the  engine.  The 
cylinders  are  placed  directly  above  the  crank  shaft  with  their 
axes  vertical.  In  the  larger  vessels  two  propellers  are  used, 
each  one  on  a  separate  shaft  driven  by  a  separate  engine. 

There  is  no  need  for  a  governor  on  a  marine  engine  as  the  resist- 
ance offered  by  the  water  to  the  revolving  propeller  increases  as 
the  speed  increases  and  this  prevents  the  engine  from  racing. 
Slower  speeds  are  secured  by  partly  closing  a  throttle  valve  which 
reduces  the  pressure  of  the  steam  supplied  to  the  cylinders. 


CHAPTER  III 
PARTS  OF  THE  STEAM  ENGINE 

The  Frame. — The  frame  of  an  engine  supports  all  of  the  work- 
ing parts  and  holds  them  in  their  proper  relative  positions. 
The  form  of  the  frame,  especially  on  the  larger  sizes  of  engines, 
is  determined  by  the  type  of  engine  and  the  purpose  for  which  it  is 
to  be  used ;  thus  a  Corliss  engine  used  on  rolling  mill  work  would 
have  an  entirely  different  kind  of  frame  than  would  the  same  type 
of  engine  when  used  for  general  power  purposes,  such  as  supplying 
power  for  a  factory. 

The  frame  of  the  automatic  high  speed  engine  is  the  simplest 
of  all  engine  frames.  This  frame,  as  shown  in  Figs.  10  and  16, 
is  made  to  rest  on  a  cast-iron  sub-base  instead  of  on  a  masonry 
foundation,  hence  the  bottom  edge  of  the  frame  is  made  in  the 
form  of  a  rectangle  to  fit  the  base.  As  most  of  these  engines  are 
of  the  center  crank  type,  the  whole  frame  is  of  rectangular  shape, 
but  is  smaller  at  the  top  than  at  the  bottom,  as  this  shape  is  best 
adapted  to  supporting  the  bearings  at  each  side,  and  to  leaving 
space  between  them  for  the  crank.  Such  engines  usually  employ 
splash  lubrication  for  the  crank  pin  and  crosshead,  that  is,  oil 
is  placed  inside  the  frame  so  that  the  crank  can  splash  into  it 
at  each  revolution  and  throw  some  of  the  mixture  on  the  rubbing 
parts.  This  makes  it  necessary  to  shape  the  frame  so  as  to 
contain  the  oil  and  also  to  cover  the  guides,  connecting  rod,  and 
cranks  to  prevent  the  oil  from  being  splashed  out.  The  rectan- 
gular shape  of  the  frame  is  well  adapted  to  splash  lubrication, 
as  the  frame  itself  makes  a  trough  for  containing  the  oil  by  merely 
extending  the  casting  across  the  bottom.  The  cast-iron  bottom 
of  the  frame  serves  not  only  to  contain  the  oil  but  strengthens 
the  frame  laterally.  Even  when  splash  lubrication  is  not  em- 
ployed, provision  must  be  made  for  catching  the  oil  that  drips 
from  the  various  bearings  as  it  would  soon  destroy  masonry 
foundations  or  soak  into  wooden  floors  around  the  engine. 

The  cylinder  of  the  high  speed  engine  usually  overhangs  the 
frame,  hence  some  provision  must  be  made  for  fastening  it  to  the 
frame.  This  is  done  by  a  ring  of  bolts  at  the  crank  end  of  the 

27 


28  STEAM  ENGINES 

cylinder.  These  bolts  are  not  used  for  aligning  the  cylinder  with 
the  frame  and  do  not  act  as  dowel  pins;  they  are  used  only  to 
hold  the  cylinder  close  up  to  the  frame.  The  cylinder  is  aligned 
by  means  of  a  projection  which  fits  accurately  into  a  bored  recess. 
In  most  cases  the  shoulder  is  on  the  frame  and  the  recess  is 
in  the  cylinder,  but  sometimes  this  arrangement  is  reversed, 
the  shoulder  being  on  the  cylinder  and  the  recess  bored  in  the 
frame.  The  bolts  for  holding  the  cylinder  may  be  outside  the 
frame,  or  they  may  be  inside  and  just  at  the  end  of  the  guides. 
The  outside  bolts  are  to  be  preferred  because  they  are  easier 
to  reach  in  case  the  cylinder  is  to  be  removed  for  repairs. 

The  frame  of  the  Corliss  engine  has  experienced  decided 
changes  in  shape  within  recent  years.  Formerly  the  girder  frame, 
as  shown  in  Fig.  17,  was  most  commonly  used.  The  girder  frame 
takes  its  name  from  the  fact  that  the  part  of  the  frame  between 
the  main  bearings  and  the  cylinder  does  not  rest  directly  on  the 
foundation  but  acts  as  a  girder,  supported  by  a  stand  or  legs 
placed  about  the  middle  of  its  length.  The  cross  section  of  the 
girder  frame,  between  the  guides  and  the  main  bearings,  is  in  the 
shape  of  the  letter  T  to  give  it  greater  stiffness  for  the  weight  of 
metal  in  it.  The  guides  form  a  part  of  the  frame  itself,  being 
either  bored  to  a  circular  shape  or  planed  to  a  V-shape,  hence  this 
part  of  the  frame  is  stiffened  by  the  guides.  The  girder  frame 
has  the  advantage  of  being  light,  and  for  small  engines,  particu- 
larly where  the  load  is  fairly  uniform,  it  is  very  satisfactory; 
but  for  heavy  and  widely  varying  loads  a  more  rigid  and  stronger 
frame  is  desirable. 

The  latter  class  of  service  has  brought  into  use  on  the  larger 
sizes  of  Corliss  engines  the  " heavy  duty"  frame,  one  form  of  which 
is  shown  in  Fig.  18.  This  frame  is  built  in  one  piece  from  the 
cylinder  to  the  main  bearings  and  is  box  shaped  to  allow  it  to  rest 
squarely  on  the  foundation  throughout  its  length,  but  it  is  cut 
away  on  the  outside  from  the  guides  towards  the  main  bearing. 
The  cylinder  is  bolted  to  the  end  of  the  frame  and  is  supported 
independently.  The  guides  are  formed  in  the  frame  itself  and 
the  frame  is  formed  into  a  complete  circle  at  both  ends  of  the 
guides  to  give  greater  strength  and  stiffness.  In  some  heavy 
duty  frames,  the  part  forming  the  guides  is  made  separate  from 
the  part  containing  the  main  bearing,  being  made  in  the  shape 
of  a  barrel  and  bolted  to  the  rear  section  and  to  the  cylinder,  but 
not  resting  directly  on  the  foundation. 


PARTS  OF  THE  STEAM  ENGINE 


29 


30 


STEAM  ENGINES 


The  frames  of  vertical  engines  are  usually  A-shaped,  as  shown 
in  Fig.  19.  In  the  larger  sizes  they  are  made  in  two  parts,  the 
upper  part  or  guide  section  being  in  one  piece  and  bolted  to  the 
lower  part  or  housing.  The  bottom  of  the  frame  is  usually  a 
rectangle  in  shape  to  permit  it  to  rest  squarely  on  the  foundation. 
A  web  is  cast  entirely  across  the  bottom  to  catch  oil  and  to 
prevent  it  from  soaking  into  the  foundation.  The  cylinders  are 
supported  by  the  guide  section  and  are  bolted  to  it.  In  some 
marine  engines,  which  are  always  of  the  vertical  type,  the  A- 
shaped  frame  is  modified,  the  cylinders  being  supported  directly 
on  steel  columns  and  the  guides  bolted  between  them.  This 
construction  is  used  to  secure  lightness  with  strength. 

The  Cylinder. — A  20"  X  24"  cylinder  of  a  locomotive  is  shown 
in  Fig.  20.  This  one  was  chosen  here  because  it  is  the  cylinder 
of  a  plain  slide  valve  engine,  and,  with  its  long  ports  and  short 


FIG.  18. 

valve  face  represents  one  extreme;  the  short-ported  Corliss 
engine  is  the  other  extreme.  The  cylinder  body  is  a  continuous 
shell  except  where  the  ports  cut  through  it;  and  around  each  end 
there  is  a  stout  flange  to  which  the  cylinder  heads  are  bolted. 

It  will  be  noticed  that  the  cylinder  is  bored  out  to  two  different 
diameters,  the  ends  being  from  ^£"  to  J^"  larger  in  diameter 
than  the  central  portion.  The  central  portion  is  called  the 
"bore"  and  the  larger  end  portion  the  "  counter  bore."  The 
length  of  the  bore  is  such  that  the  piston  rings  slightly  overtravel 
it  in  order  to  prevent  wearing  shoulders  in  the  bore.  The 
counterbore  is  made  large  enough  to  allow  reboring  the  cylinder 
two  or  three  times  to  bring  it  back  to  a  true  cylindrical  form  after 
it  has  become  worn  by  the  piston. 

The  heads  are  cast  with  a  shoulder  which  is  turned  to  a  close 
fit  with  the  counterbore,  and  they  are  then  bolted  on.  The  head 
end  head  is  recessed  to  receive  the  nut  which  holds  the  piston  on 
the  piston  rod,  thus  reducing  the  necessary  clearance;  and  the 


PARTS  OF  THE  STEAM  ENGINE  31 


FIG.  19. 


32 


STEAM  ENGINES 


crank  end  is  recessed  on  the  outside  to  form  a  stuffing  box.  In 
this  case,  both  heads  are  made  thin,  and  ribs  are  cast  on  the 
outside  of  them  to  give  greater  strength.  The  walls  of  the 
cylinder  are  practically  uniform  in  thickness,  and,  in  order  to 
accomplish  this,  recesses  are  left  on  the  outside  of  the  cylinder 
where  necessary.  Uniform  thickness  of  the  walls  of  a  steam 
cylinder  is  desirable  in  order  to  prevent  unequal  strains  in  the 
cylinder,  due  to  expansion  caused  by  the  heat  to  which  it  is 
subjected. 

That  part  of  the  casting  forming  the  valve  chest  and  steam 
passages  is  much  more  complicated  than  the  other  parts  of  the 


FIG.  20. 

cylinder  on  account  of  the  necessary  provision  for  fastening  the 
cylinder  to  the  saddle,  and  conducting  the  live  steam  to  it  and 
the  exhaust  steam  away  from  it.  The  valve  seat  consists  'of  a 
raised  rectangular  table  with  the  steam  and  exhaust  ports  leading 
up  to  its  surface.  The  steam  passage  enters  through  the  saddle 
and  then  divides,  entering  the  bottom  of  the  steam  chest  on  each 
side  of  the  valve  seat  at  A  and  A.  The  exhaust  passage  lies 
between  the  steam  passages  and  it  is  of  such  width  that  a  web  B 
is  cast  across  it  to  brace  its  side  walls. 

The  entire  outside  of  the  cylinder  and  valve  chest  is  covered 
with  planished  sheet  iron  to  give  a  smoother  and  neater  appear- 


PARTS  OF  THE  STEAM  ENGINE 


33 


ance  to  it.  The  space  around  the  barrel  of  the  cylinder,  between 
the  cylinder  and  covering,  is  filled  with  nonconducting  material 
such  as  asbestos,  to  reduce  the  loss  of  heat  by  radiation. 

The  cylinder  shown_in  Fig.  £1  represents  a  usual  type  of  con- 
struction for  automatic  high  speed  engines  having  flat  valves. 
As  compared  with  the  cylinder  shown  in  Fig.  20,  this  one  has  a 
larger  diameter  in  proportion  to  its  length,  has  shorter  ports 
due  to  the  greater  width  of  valve,  and  the  cylinder  casting  is 
somewhat  simpler.  The  cylinder  is  counterbored,  as  is  the  case 
with  all  steam  engine  cylinders,  and  in  this  case  the  heads  are 


FIG.  21. 

recessed  to  allow  the  ports  to  end  behind  the  piston  instead  of 
ending  flush  with  the  cylinder  walls.  The  heads  are  set  into  the 
cylinders  and  bolted  on.  On  account  of  the  different  methods 
of  fastening  the  piston  to  the  piston  rod,  the  head  end  head  is 
not  recessed  but  forms  a  plane  surface  to  agree  with  the  face  of 
the  piston.  This  head  is  made  double  with  an  air  space  between 
for  insulation.  The  crank  end  head  is  recessed  on  the  outside 
for  the  stuffing  box,  which  extends  slightly  into  the  cylinder. 
The  piston  is  cored  out  to  fit  the  projection  on  the  head,  and  thus 
to  reduce  the  clearance.  The  clearance  is  further  reduced  by 
the  short  ports.  The  exhaust  port  in  this  cylinder  is  wide  and 
shallow,  and  its  walls  do  not,  therefore,  require  bracing. 


34 


STEAM  ENGINES 


A  Corliss  engine  cylinder  is  shown  in  Fig.  22.  The  bottom  of 
the  cylinder,  which  is  rectangular  in  shape  and  flat,  rests  on  its 
own  bedplate  and  is  bolted  to  it.  The  two  admission  valves  are 
placed  at  the  top  of  the  cylinder  and  the  two  exhaust  valves  at 
the  bottom,  all  being  at  the  ends  of  the  cylinder.  The  axes 
of  the  valves  are  placed  across  the  cylinder,  and  the  valve  cham- 
bers are  of  rectangular  form  so  that  the  cylinder  has  a  square 
cornered  appearance.  In  later  types  of  engines  the  tendency 
is  to  round  the  valve  chambers  to  conform  to  the  shape  of  the 
valve,  as  shown  in  Fig.  11.  The  steam  chamber  S  is  cored  out  of 
the  top  of  the  cylinder  and  extends  all  the  way  across  it,  giving 
a  flat  top  to  the  cylinder.  As  mis  chamber  contains  steam  at 
boiler  pressure,  webs  are  cast  in  it  to  give  a  greater  strength. 


FIG.  22. 

The  steam  chamber  is  formed  right  upon  the  walls  of  the  cylinder 
so  it  may  act  as  a  steam  jacket  to  this  part  of  the  cylinder. 
The  exhaust  chamber,  E,  which  contains  steam  at  a  lower  tem- 
perature, is  separated  from  the  cylinder  walls  by  a  cored  chamber. 
The  exposed  portions  of  the  valve  chambers  are  usually  polished 
to  decrease  the  radiation  of  heat,  and  the  other  parts  of  the 
cylinder  are  covered  with  nonconducting  material. 

In  the  cylinder  shown  here  the  heads  are  cast  very  thin  and  are 
strengthened  by  webs  cast  on  the  outside  of  them,  the  head  end 
head  being  covered  by  a  cover  plate  to  give  a  neater  appearance. 
Both  heads  are  cast  with  plain  inner  surfaces,  and  both  faces 
of  the  piston  are'  made  flat  to  correspond.  This  allows  the  piston 
to  be  brought  very  close  to  the  cylinder  heads  and  thus  reduce 
the  clearance.  The  steam  and  exhaust  ports  are  partly  cored 


PARTS  OF  THE  STEAM  ENGINE 


35 


out  of  the  heads  so  there  may  be  some  piston  surface  exposed  to 
pressure  when  the  piston  is  at  the  end  of  its  stroke. 

The  placing  of  the  exhaiist  valves  in  a  Corliss  engine  at  the 
lowest  point  of  the  cylinder  allows  water  to  drain  through  them 
when  the  cylinder  is  being  warmed  up  preparatory  to  starting 
the  engine.  In  a  slide  valve  engine  the  valve  is  placed  at  the 
side  or  on  top  of  the  cylinder,  and  drain  valves  or  cocks  must  be 
placed  in  the  bottom  of  the  cylinder. 

The  cylinder  of  a  four  valve  high  speed  engine,  shown  in  Fig. 
23,  is  constructed  the  same  as  a  Corliss  cylinder,  the  principal 
difference  being  the  greater  diameter  in  proportion  to  the  length. 


FIG.  23. 

The  ports  and  valves  of  these  engines  are  often  made  double  in 
order  to  secure  a  greater  opening  with  a  small  valve  travel, 
which  is  desirable  when  the  speed  of  the  valve  is  high.  The 
clearance  in  the  cylinders  of  these  engines  is  relatively  greater 
than  that  in  the  Corliss  engine  because,  while  the  piston  may  be 
brought  as  close  to  the  head,  the  volume  between  the  piston  and 
head  will  be  greater  because  of  the  larger  diameter.  In  some 
makes  of  four  valve  high  speed  engines  and  also  in  some  large 
Corliss  engines  an  effort  is  made  to  reduce  the  clearance  to  a 
minimum  by  placing  the  valve  chambers  in  the  cylinder  heads, 
as  shown  in  Fig.  24 ;  this  reduces  somewhat  the  length  of  ports  and 
thereby  reduces  the  clearance. 


36 


STEAM  ENGINES 


The  cylinder  of  a  large  marine  engine  is  shown  in  Fig.  25.  The 
peculiar  feature  about  this  cylinder  is  the  shape  of  the  heads  and 
the  fact  that  the  cylinder  is  fitted  with  a  liner.  The  piston  is 


FIG.  24. 


cone-shaped  to  conform  to  the  shape  of  the  heads,  this  shape  being 
adopted  to  aid  the  drainage  of  water  of  condensation  into  the 
ports.  Having  a  liner  in  the  cylinder,  as  indicated  at  A ,  simplifies 


FIG.  25. 


the  work  of  casting  the  cylinder  since  the  liner  is  cast  separately, 
and  permits  of  a  sound  and  close  grained  liner  being  obtained  while 
the  cylinder  proper  is  made  of  softer  iron.  The  liner  is  fastened 


PARTS  OF  THE  STEAM  ENGINE 


37 


to  the  cylinder  at  the  bottom  by  sunk  head  bolts  in  the  inward 
projecting  flange,  the  top  being  turned  to  a  close  fit.  The  ports 
may  be  either  cut  through  the^ liner  or  carried  around  it;  in  this 
case  they  are  carried  around  it.  In  order  to  secure  lightness  and 
strength,  the  heads  are  cast  with  double  wall  chambers  which  are 
strengthened  by  webs.  This  method  of  construction  also  reduces 
loss  of  heat  from  the  cylinders  since  there  is  an  enclosed  air  space 
next  to  the  cylinder  walls. 

The    Piston. — An    engine    piston    must    meet    the    following 


FIG.  20. 

conditions :  it  must  have  enough  strength  to  withstand  the  steam 
pressure  acting  upon  it  and  yet  be  no  heavier  than  necessary, 
as  its  weight  controls  its  inertia  and  causes  it  to  wear  the  cylinder, 
especially  if  the  cylinder  is  horizontal;  it  must  have  a  broad  rim 
or  working  face,  especially  in  horizontal  engines,  in  order  to  have 
plenty  of  rubbing  surface  and  reduce  wear;  it  must  be  constructed 
to  prevent  the  leakage  of  steam  past  it.  The  last  result  is  secured 
by  having  the  piston  a  little  smaller  than  the  bore  of  the  cylinder 
and  closing  the  gap  between  piston  and  cylinder  by  means  of 
rings  sprung  into  grooves  in  the  piston. 

The  box  piston  is  by  far  the  most  common  type  used  in  cylin- 
ders up  to  24  inches  in  diameter.     One  of  the  simplest  forms  of 


38 


STEAM  ENGINES 


box  piston  is  shown  in  Fig.  26.  This  piston  consists  of  a  simple 
hollow  casting  with  flat  faces  of  uniform  thickness  on  both 
sides  and  with  a  hub  cast  into  its  center  for  receiving  the  piston 
rod.  The  piston  is  strengthened  by  casting  webs  across  it  so  as 
to  divide  it  into  a  number  of  compartments.  A  hole  is  cast  into 
each  of  the  compartments  through  which  the  core  may  be  re- 
moved, after  which  it  is  drilled  and  tapped  to  receive  a  plug. 
In  order  to  fasten  the  piston  rod  in  the  piston,  the  hub  of  the 
piston  is  bored  smaller  than  the  rod,  and  the  end  of  the  rod  is 
turned  to  fit  the  hub  which  rests  against  the  shoulder  on  the 
rod.  The  two  are  then  fastened  together  with  a  countersunk 


FIG.  26a. 

nut  screwed  on  the  rod,  the  outer  end  of  the  nut  being  flush  with 
the  face  of  the  piston. 

This  piston  is  supplied  with  a  single  heavy  packing  ring  placed 
at  the  center.  It  is  common,  however,  to  place  two  lighter  rings 
on  the  piston,  one  near  each  edge.  In  some  cases  the  piston  is 
supplied  with  four  light  rings  in  two  grooves  placed  near  each 
edge.  The  rings  are  turned  out  of  cast  iron  and  are  made  a  little 
larger  in  diameter  than  the  bore  of  the  cylinder.  They  are 
then  sawed  through  diagonally,  only  one  cut  being  in  a  ring,  and 
the  ring  is  then  sprung  on  the  piston.  The  spring  in  the  ring, 
since  it  is  larger  than  the  cylinder,  keeps  it  pressed  outward  against 
the  walls  of  the  cylinder  and  prevents  steam  from  leaking  past 
it.  When  a  single  ring  is  used,  a  clip  is  placed  at  the  cut  in  the 
ring  to  prevent  steam  from  leaking  through,  as  shown  in  Fig. 
26a,  but  when  the  piston  has  more  than  one  ring  no  clip  is  used 


PARTS  OF  THE  STEAM  ENGINE 


39 


the  rings  being  simply  placed  on  in  such  manner  as  to  break  joints. 
Instead  of  the  faces  of  the  box  piston  being  flat,  they  are  often 


FIG.  27a. 

shaped  to  conform  to  projections  on  the  inside  of  the  cylinder 
heads. 

Two  examples  of  locomotive  pistons  are  shown  in  Fig.  27.  In 
both  of  these  designs  a  special  effort  is  made  to  secure  strength 
and  lightness.  The  one  shown  at  A  is  made 
entirely  of  cast  iron  of  a  simple  T  section  and 
with  two  packing  rings  in  the  rim.  This  piston 
is  forced  on  the  tapered  end  of  the  piston  rod  by 
means  of  a  nut.  The  piston  shown  at  B  is 
made  in  two  parts,  a  central  conical  part  made 
of  steel,  and  a  cast-iron  rim  which  is  bolted  to 
the  central  portion  by  means  of  countersunk  bolts. 
A  peculiar  feature  of  this  piston  is  that  the  bottom, 
which  carries  the  weight,  has  a  much  broader 
wearing  surface  than  the  top.  This  piston  is 
pressed  against  a  shoulder  on  the  piston  rod  by 
means  of  a  nut  screwed  on  the  end  of  the  rod. 
The  end  of  the  rod  has  a  slight  taper  to  secure 
a  better  connection  between  the  rod  and  the 
hub. 

The  pistons  of  vertical  engines,  especially  of 
the  marine  type,  are  made  as  light  as  possible 
consistent  with  proper  strength.  A  wide  rim  or  wearing  surface 
is  not  necessary  on  these  pistons,  since  the  weight  of  the 
piston  is  not  carried  by  the  cylinder.  One  type  of  marine 
engine  piston  is  shown  in  Fig.  28.  This  piston  is  constructed 


FIG.  28. 


40 


STEAM  ENGINES 


with  a  steel  web,  and  the  entire  rim  is  made  of  two  cast-iron  spring 
rings  which  are  held  in  place  by  a  follower  ring  bolted  to  the  steel 
web. 

Corliss  engine  pistons  are  usually  of  the  "built  up"  type,  con- 
sisting of  several  adjustable  parts  bolted  together.  A  piston  of 
this  type  is  illustrated  in  Fig.  29.  The  ribbed  body  of  this 
piston,  in  which  the  rod  is  fastened,  is  called  a  spider.  The  rim  is 
made  in  two  parts,  2  and  3,  and  is  called  a  "bull  ring"  or  "junk 
ring."  This  carries  a  single  heavy  pack- 
ing ring.  The  bull  ring  is  held  in  place 
by  a  follower  plate,  4,  which  is  fastened 
by  tap  bolts  to  the  spider.  The  bull 
ring  forms  the  wearing  surface  of  the 
piston  and  can  be  adjusted  by  set  screws 
so  as  to  make  the  axis  of  the  piston 
agree  with  that  of  the  cylinder.  The 

_ 1  P*^       ;Kxx/-°txEi        bull  ring  is  made  in  two  parts  so  that 

\  ]  |         the  packing  ring  may  be  put  in  with- 

(  .'        j      1 1         out  having  to  spring  it  over  the  piston 

and  also  because  access  may  be  had  to 
the  packing  ring  without  removing  the 
piston  from  the  bore  of  the  cylinder. 
The  piston  rod  has  a  taper  fit  against 
a  collar  and  is  riveted  over  a  heavy  washer 
at  the  end,  after  which  a  key  is  passed 
through  both  rod  and  hub. 

Stuffing  Box. — Stuffing  boxes  are  used 
to  prevent  the  leakage  of  steam  at  the 
points  where  the  piston  rod  and  valve  rod 
pass  through  the  cylinder  head  and  valve  chest,  respectively.  As 
the  stuffing  boxes  at  both  of  these  points  are  constructed  alike,  a 
description  of  one  will  be  sufficient.  A  stuffing  box  consists  of 
two  parts:  first,  an  annular  space  surrounding  the  rod,  as  shown 
in  Fig.  30;  and,  second,  a  cover  plate  called  a  "gland"  extending 
into  the  stuffing  box  in  such  manner  as  to  compress  the  packing 
material  when  it  is  screwed  down.  Two  stud  bolts  are  screwed 
into  the  cylinder  head,  one  on  each  side  of  the  stuffing  box,  and 
these  extend  through  the  gland  and  end  in  a  nut  so  the  gland 
may  press  upon  the  packing.  The  bottom  of  the  stuffing 
box  is  most  often  cut  away  at  an  angle,  as  shown  in  Fig.  30,  but 
sometimes  it  is  made  flat.  The  hole  through  which  the  rod 


FIG.  29. 


PARTS  OF  THE  STEAM  ENGINE 


41 


passes  into  the  cylinder  must  be  large  enough  to  accommodate 
any  lack  of  alignment  between  the  rod  and  cylinder  but  must  not 
be  large  enough  to  allow  any  of  the  packing  material  to  be 
squeezed  through  it.* 

Formerly,  braids  or  strands  of  hemp  soaked  in  tallow  were 
wrapped  around  the  piston  rod  and  pressed  into  the  stuffing  box 
or  packing,  but,  as  increasing  steam  pressures  and  temperatures 
became  common,  this  kind  of  packing  became  unsatisfactory. 
There  are  now  a  great  variety  of  packings  on  the  market  made  of 
vegetable  fiber,  asbestos,  or  rubber  in  various  combinations,  and 
frequently  mixed  with  graphite  for  a  lubricant,  these  being  made 
of  sizes  and  shapes  to  fit  neatly  into  the  stuffing  box.  Woven 


FIG.  30. 

packing  is  often  made  square  in  cross  section  and  divided  diago- 
nally into  two  parts  so  that  when  the  gland  is  screwed  down  the 
packing  is  pushed  out  squarely  against  the  rod. 

Metallic  packing  is  often  used  in  the  stuffing  boxes  of  steam 
engines,  but  requires  a  good  alignment  of  the  piston  rod  to  work 
properly.  Metallic  packing  is  usually  made  in  the  form  of 
babbitt  metal  rings,  which  are  pressed  against  the  piston  rod, 
preventing  the  leakage  of  steam  and  causing  but  little  friction. 
An  example  of  this  kind  of  packing  is  shown  in  Fig.  31.  The 
gland,  G,  is  merely  a  heavy  cover  plate  made  tight  by  a  copper 
wire  acting  as  a  gasket.  The  ring  1  presses  against  the  casing 
2  and  forces  the  babbitt  metal  rings  3,  4,  and  5  against  the  rod. 
These  rings  are  made  in  segments  and  placed  so  as  to  break  joints. 


42 


STEAM  ENGINES 


A  follower  ring  6  is  held  in  place  by  a  heavy  spring  and  keeps  the 
rings  in  their  proper  position,  but  the  spring  is  not  depended  upon 
to  press  the  packing  against  the  rod.  This  is  done  by  the  steam 
pressure  acting  behind  the  ring  6  so  that  the  tightness  of  the 
packing  varies  with  the  steam  pressure. 

The  Crosshead. — The  crosshead  moves  in  a  straight  line 
between  guides  and  is  for  the  purpose  of  joining  the  piston  rod  to 
the  connecting  rod.  It,  therefore,  has  two  joints :  a  stationary  one 


FIG.  31. 

between  the  piston  rod  and  crosshead;  and  a  pin  joint  between 
the  connecting  rod  and  the  crosshead  so  the  connecting  rod  may 
be  free  to  move. 

There  are  three  general  types  of  crossheads  used  upon  station- 
ary engines,  called  respectively,  the  "wing,"  the  "block," 
and  the  "slipper"  crossheads.  The  wing  crosshead  is  illustrated 
by  Fig.  32.  It  consists  of  a  heavy  steel  or  cast-iron  block  forming 
three  sides  of  a  rectangle  and  having  a  heavy  "wrist  pin"  passing 
between  the  side  pieces  or  wings.  The  piston  rod  is  threaded 
at  the  end  and  screwed  into  the  front  crossbar  of  the  crosshead, 


PARTS  OF  THE  STEAM  ENGINE 


43 


being  held  securely  by  means  of  a  lock  nut.  The  wings  form  the 
rubbing  surfaces,  and,  to  reduce  friction,  they  are  often  con- 
structed with  grooves  filled  with  babbitt  metal.  The  guides  consist 
of  four  flat  bars  between  whicti  the  wings  move.  The  guides  are 
adjustable  up  and  down  to  take  up  wear  in  the  crosshead.  The 
wrist  pin  is  usually  made  of  steel,  separate  from  the  crosshead, 
and  is  held  in  place  by  nuts  on  the  ends,  or  sometimes  by  means  of 


FIG.  32. 

a  set  screw.     The  wing  crosshead  is  often  used  on  the  smaller 
sizes  of  engines,  especially  those  of  the  plain  slide  valve  type. 

The  block  crosshead  is  most  often  found  on  Corliss  and  auto- 
matic high  speed  engines.  As  shown  in  Fig.  33,  it  consists  of  a 
heavy  cast-iron  block  with  the  wrist  pin  passing  through  its 
center  and  with  the  piston  rod  screwed  or  keyed  into  the  center 
at  the  front.  The  rubbing  surfaces,  located  at  top  and  bottom, 
are  of  circular  shape  for  bored  guides,  and  V-shaped  for  planed 
guides.  Both  top  and  bottom  rubbing  surfaces  or  "shoes" 
are  adjustable  and  have  babbitt  metal  inserted  in  grooves  or 


44 


STEAM  ENGINES 


holes  to  reduce  friction.  With  this  type  of  crosshead  the  guides 
are  not  adjustable  but  the  shoes  of  the  crosshead  are  adjusted 
to  take  up  wear  and  to  bring  the  crosshead  into  alignment 
with  the  piston  rod.  The  shoes  are  adjusted  by  means  of 
wedges  placed  between  the  shoes  and  the  body  of  the  crosshead 
and  moved  by  screws  fitted  with  lock  nuts  to  hold  the  wedge  in 


FIG.  33. 

position  after  adjustment.  The  method  of  securing  the  wrist 
pin  in  this  crosshead  is  shown  by  the  taper  ends  of  the  pin  com- 
bined with  a  nut  on  the  end.  The  method  of  carrying  oil  to 
the  rubbing  surface  of  the  pin  is  clearly  shown. 


FIG.  33a. 

The  slipper  type  of  crosshead,  Fig.  33a,  resembles  the  wing 
type,  but  differs  from  it  in  having  the  wrist  pin  and  main  body 
of  the  crosshead  placed  above  the  wings.  In  this  case  the  wings 
are  comparatively  thin  but  the  rubbing  surface,  which  is  all  of  the 
bottom  of  the  crosshead,  is  broad.  The  guides  consist  of  a 
flat  planed  surface  on  the  engine  frame,  with  a  rectangular  bar 


PARTS  OF  THE  STEAM  ENGINE 


45 


at  each  side  to  fit  on  top  of  the  slipper.  These  rectangular 
bars  are  adjustable  to  take  up  the  wear  of  the  crosshead.  The 
wrist  pin  consists  of  a  simple  steel  cylinder  and  is  clamped 
between  the  jaws  of  the  crosshead,  which  are  split  and  provided 
with  bolts  for  this  purpose.  The  wrist  pin  is  arranged  so  it  may 
be  turned  through  90  degrees  as  it  wears,  thus  keeping  it  round. 

Connecting  Rods. — The  connecting  rod  connects  the  wrist 
pin  and  crank  pin  and  serves  to  transmit  the  force  acting  upon  the 
piston  to  the  crank  pin.  For  one-half  of  a  revolution  of  the  fly- 
wheel the  forces  acting  along  the  connecting  rod  are  pushing 
and  for  the  other  half  of  the  revolution  they  are  pulling.  The 
connecting  rod  consists  of  an  adjustable  bearing  at  each  end 
connected  by  the  shank  or  rod  proper. 

The  cross  section  of  the  rods  is  made  in  various  shapes,  depend- 
ing upon  the  type  of  engine  with  which  they  are  to  be  used.  For 
slow  running  engines  of  the  Corliss  type  the  rod  is  usually  round, 


fTTL 


FIG.  34. 

being  largest  at  the  middle  and  tapering  towards  the  ends. 
With  engines  of  higher  speed  the  rod  is  often  shaped  like  a  long 
cone,  tapering  towards  the  crank  end  and  flattened  on  the  sides 
so  as  to  approach  a  rectangular  cross  section  as  the  diameter 
increases.  High  speed  stationary  engines  and  some  locomotives 
have  a  rod  of  rectangular  cross  section  increasing  in  depth 
towards  the  crank  end.  Many  of  these  engines  have  the  cross 
section  of  the  rod  of  an  I-shape  in  order  to  make  them  light  and 
strong.  Marine  engines  usually  have  round  rods. 

Great  variety  is  found  in  the  construction  of  the  ends  of  the 
connecting  rod,  but  they  will  usually  fall  in  one  of  the  three 
general  classes  called  respectively  the  "box"  or  "solid  end," 
the  "strap  end,"  or  the  "marine  end."  Often  the  rod  will  have 
one  end  of  the  "box"  type  and  the  other  end  of  the  "strap" 
type.  The  box  or  solid  end  type  of  construction  is  well  illustrated 
in  Fig.  34,  which  shows  that  the  end  of  the  rod  is  flattened  and 
has  a  rectangular  slot  milled  into  it.  Into  this  slot  are  placed 
the  two  halves  of  the  bearings,  which  are  made  of  brass  or  bronze. 


46  STEAM  ENGINES 

These  halves  are  separated  by  a  small  space  to  allow  them  to 
be  brought  closer  together  as  they  wear  away.  The  " brasses'7 
are  cast  with  flanges  at  the  sides  which  fit  the  sides  of  the  rod  to 
prevent  their  movement  sidewise.  Behind  one  of  the  brasses 
is  placed  a  wedge-shaped  block  with  a  screw  held  by  nuts  at 
each  end  passing  through  it.  Adjustment  of  the  brasses  is  made 
by  turning  this  screw  and  moving  the  wedge-shaped  block  up- 
ward, thus  forcing  the  brasses  closer  together.  By  having  one 
'of  the  adjusting  wedges  placed  on  the  inside  of  the  end  and  the 
other  placed  on  the  outside  of  the  end,  the  length  of  the  connect- 
ing rod,  which  is  the  distance  from  the  center  of  the  wrist  pin 
to  the  center  of  the  crank  pin,  is  not  changed  when  both  ends 
are  adjusted  at  the  same  time. 

A  strap  end  connecting  rod  is  illustrated  in  Fig.  35.     In  this 
form,  the  connecting  rod  ends  at  the  brasses  and  a  separate  steel 


FIG.  35. 

strap  passes  around  the  brasses  and  laps  over  the  end  of  the  rod 
at  top  and  bottom.  The  strap  is  fastened  to  the  connecting 
rod  by  means  of  two  bolts  which  pass  entirely  through  both 
ends  of  the  strap  and  through  the  connecting  rod  and  are  secured 
by  a  nut  and  lock  nut.  Keys  are  inserted  between  the  strap  and 
connecting  rod  at  top  and  bottom  to  keep  the  strap  in  line  with 
the  rod  and  to  relieve  the  bolts  of  shear  and  permit  the  use  of 
lighter  bolts.  The  brasses  are  adjusted  by  means  of  a  wedge  in 
back  of  one  of  the  brasses,  the  wedge  being  moved  up  or  down  by 
a  bolt  threaded  into  the  brass  and  passing  through  the  strap  with 
a  lock  nut  on  the  outside. 

Strap  end  connecting  rods  are  used  commonly  oh  locomotives 
but  are  dropping  out  of  use  on  stationary  engines.  When  used 
on  locomotives,  the  brasses  form  the  bearing  against  the  pins, 
but  for  stationary  engines  the  brasses  are  usually  lined  with 
babbitt  metal.  Locomotive  connecting  rods  have  the  brasses 
adjusted  by  means  of  a  wedge  driven  down  behind  one  side  of 
the  brass  and  locked  in  place  by  means  of  a  set  screw. 

The  right  hand  end  of  the  connecting  rod  shown  in  Fig.  36 


PARTS  OF  THE  STEAM  ENGINE 


47 


is  of  the  marine  type;  the  left-hand  end  is  of  the  box  type, 
described  before.  Marine  end  connecting  rods  are  used  on  all 
marine  engines  and  on  some  vertical  stationary  ones.  The 
example  shown  here  is  from  a  ftationary  engine.  Those  designed 
for  marine  engines  usually  have  both  ends  of  the  connecting 
rod  of  the  marine  type,  instead  of  only  one.  In  the  marine  end 
connecting  rod,  adjustment  of  the  brasses  is  secured  by  means 


FIG.  36. 

of  bolts  placed  parallel  to  the  connecting  rod  and  passing  through 
shoulders  on  the  rod  and  on  the  removable  end,  as  shown.  This 
form  of  adjusting  device  permits  a  short  end  for  the  rod,  which 
accounts  for  its  common  use  on  marine  engines,  where  the  crank 
pin  passes  close  to  the  floor. 

Crank  and  Crank  Pin. — The  steam  pressure  acting  upon  the 
piston  is  transmitted  through  the  connecting  rod  to  the  crank 
pin  and  then  through  the  crank  to  the  shaft.  Engines  may  be 
divided  into  overhung  crank  engines,  in  which  the  crank  is  at 
the  end  of  the  shaft;  and  into  center  crank  engines,  in  which 
the  crank  is  placed  at  or  near  the  center  of  the  shaft. 

An  overhung  crank  for 
an  engine  of  medium  size 
is  shown  in  Fig.  37.  This 
crank  is  made  in  the  form 
of  a  cast-iron  disk,  with 
holes  bored  to  receive  the 
crank  pin  and  the  shaft. 
The  crank  disk  is  made 
thicker  opposite  the  crank 
pin  .than  it  is  on  the 
crank  pin  side  in  order  to  counterbalance  it.  The  crank  disk 
is  either  forced  on  the  shaft  and  keyed,  as  shown  here,  or 
else  shrunk  on,  in  which  case  a  key  is  unnecessary.  The  crank 
pin  is  usually  forced  in  by  hydraulic  pressure  and  then  riveted 
over.  Overhung  cranks  for  large  slow  speed  engines  are  some- 
times forged  from  steel  in  the  shape  shown  in  Fig.  38.  This 


48 


STEAM  ENGINES 


shape  does  not  permit  of  as  much  counterbalancing  as  the  disk 
shape,  but,  on  the  other  hand,  slow  speed  engines  do  not  require 
as  heavy  counterweights  as  do  high  speed  engines. 

A  crank  for  a  center  crank  engine  is  shown  in  Fig.  39.     This 


9 


FIG.  38. 

consists  of  two  cast-iron  disks  with  counterweights,  fastened  to 
the  shaft,  which  is  made  in  two  sections,  and  with  the  crank 
pin  connecting  the  two  disks.  Another  form  of  center  crank  is 
illustrated  in  Fig.  40,  in  which  the  cranks,  crank  pin,  and  shaft 
are  all  in  one  piece  and  forged  from  steel.  The  counterweights 

are  made  of  cast  iron  and 
bolted  to  the  cranks  oppo- 
site the  crank  pin,  as  shown. 
Bearings. — There  aje  al- 
ways two  bearings  for  the 
shaft  on  an  engine.  In 
center  crank  engines  these 
two  bearings  are  alike,  but 
in  side  crank  engines  the 
one  next  to  the  crank,  called  the  main  bearing,  is  much  heavier 
than  the  other,  or  outer  bearing. 

The  bearings  must  not  only  support  the  weight  of  the  shaft 
and  flywheel,  but  must  resist  the  thrust  of  the  piston,  and  also 
resist  the  pull  of  the  belt,  if  there  is  one.  The  resultant  of  all 
of  these  forces  causes  the  bearings  to  wear  at  an  angle  with  the 


FIG.  39. 


PARTS  OF  THE  STEAM  ENGINE 


49 


horizontal  rather  than  at  the  bottom  and  top.  Since  the  wear 
comes  at  an  angle  to  the  horizontal,  provision  must  be  made  for 
adjusting  the  bearings  at  an  angle.  This  is  done  by  dividing  the 
bearing  into  two  parts  with  the  division  line  between  the  parts 
making  an  angle  with  the  horizontal,  or  by  dividing  the  bearing 
into  four  parts  so  that  adjust- 
ment may  be  made  at  the 
side  or  at  the  top. 

Two  part  bearings,  such  as 
are  often  found  on  high 
speed  engines,  are  illustrated 
in  Figs.  41  and  42.  In  Fig.  41 
the  bearing  is  divided  along  a 
line  making  about  45  degrees 
with  the  horizontal.  The  lower  part  has  a  flat  plate  cast  on  its 
outer  surface  which  rests  in  the  frame  and  which  may  be  adjusted 
vertically  by  shimming,  or  placing  thin  sheets  of  metal  under  it. 
The  top  half  has  one  flat  plate  at  the  side  and  another  at  the  top. 
This  part  of  the  bearing  may  be  adjusted  horizontally  by  means 
of  bolts  passing  through  the  back  of  the  frame  and  bearing  against 


FIG.  40. 


FIG.  41. 

the  flat  plate  at  the  side.  The  cap  rests  on  the  flat  plate  at  the 
top  and  is  held  in  place  by  bolts  passing  down  into  the  frame. 
The  bearing  is  lined  with  babbitt  metal  having  a  series  of 
diagonal  grooves  cut  in  it  for  distributing  the  oil  throughout  the 
length  of  the  bearing. 

The  bearing  shown  in  Fig.  42  is  similar  to  the  above  except 
for  the  method  of  adjusting.     In  this  bearing  the  top  part  is 


50 


STEAM  ENGINES 


moved  horizontally  by  means  of  a  wedge  placed  behind  it  and 
which  may  be  moved  up  or  down  by  a  bolt  extending  through  the 
cap. 


FIG.  42. 


Large  horizontal  engines  usually  have  main  bearings  of  the  type 
shown  in  Fig.  43.  In  this  type  the  bearing  is  divided  into  four 
parts,  the  side  pieces  being  adjustable.  One  of  these  side  pieces 


FIG.  43. 

is  adjusted  by  shims  and  the  other  by  set  screws  passing  through 
the  frame.     The  top  part  of  the  bearing  is  made  as  a  part  of  the 


PARTS  OF  THE  STEAM  ENGINE 


51 


cap,  which  is  held  in  place  by  bolts  passing  down  into  the  frame 
and  which  serves  to  hold  the  parts  of  the  bearing  together.  The 
bearing  is  lined  with  babbitt  metal  with  diagonal  grooves  cut  in 
it.  The  advantage  of  this  type  of  construction  is  that  the  bearing 
may  be  removed  without  removing  the  shaft,  by  taking  off  the 
cap,  slightly  lifting  the  shaft,  and  turning  the  bearing  around. 

The  Flywheel. — The  flywheel  serves  a  threefold  purpose: 
It  sometimes  serves  to  transmit  the  power  of  the  engine  to  other 
machines  by  means  of  belts;  to  store  up  enough  energy  near  the 
middle  of  the  piston  stroke  to  carry  the  engine  past  center;  and, 
by  storing  up  energy  at  one  part  of  the  stroke  and  giving  it  out 


FIG.  44. 

again  at  other  parts,  to  prevent  fluctuations  of  speed  during  a 
revolution  of  the  flywheel. 

Small  engines  are  usually  supplied  with  two  ordinary  belt 
wheels.  In  slow  speed  engines  one  of  these  wheels  is  sometimes 
larger  than  the  other,  but  in  medium  and  high  speed  engines 
both  wheels  are  of  the  same  size  and  kind.  Even  when  high  speed 
engines  are  direct  connected  to  generators  there  is  often  one 
flywheel. 

Wheels  less  than  9  feet  in  diameter  are  usually  cast  in  one 
piece,  but  with  the  hub  split  on  one  side,  as  shown  in  Fig.  44,  so 
it  may  be  clamped  to  the  shaft  by  two  bolts,  one  on  each  side  of 
the  spokes.  These  bolts  are  not  depended  upon  to  hold  the 
wheel,  however,  but  merely  to  simplify  putting  the  wheel  on  the 


52 


STEAM  ENGINES 


shaft.     The  hub  is  held  securely  to  the  shaft  by  means  of  a  close 
fitting   key. 

Flywheels  between  9  and  16  feet  in  diameter  are  commonly 
made  in  halves,  and  larger  sizes  are  divided  into  a  greater  number 
of  parts,  as  a  16  foot  piece  is  about  as  large  as  can  be  shipped  on  an 
ordinary  flat  car.  Fig.  45  illustrates  the  method  of  joining  the 
halves  of  a  large  flywheel.  The  hub  is  clamped  to  the  shaft  by 


FIG.  45. 

bolts  extending  all  the  way  through  on  each  side  of  the  shaft,  after 
which  the  hub  is  keyed  to  the  shaft.  This  type  of  wheel  is  used 
only  as  a  flywheel  and  not  as  a  belt  wheel,  hence,  the  rim  is  made 
narrow  and  deep  in  order  to  concentrate  its  weight  as  far  from  the 
center  of  the  shaft  as  possible.  The  rim  is  fastened  together  by 
inwardly  projecting  flanges  bolted  together  and  by  J-shaped  bars 
or  links  let  into  the  sides  of  the  rim.  These  links  are  machined 
exactly  to  length  between  heads  and  the  slots  similarly  machined. 
The  links  are  made  shorter  than  the  slots  by  from  one  in  one 
thousand  to  one  in  eight  hundred.  The  links  are  then  heated 
and  expanded  until  they  will  go  into  the  slots.  Upon  cooling, 
the  links  contract  and  hold  the  halves  of  the  rim  together  tightly. 


* 

;i. 
CHAPTER  IV 

HEAT,   WORK,   AND   PRESSURE 

Force. — If  a  weight  is  held  in  the  outstretched  hand,  a  down- 
ward pull  on  the  hand  will  be  experienced.  Unless  this  pull  is  re- 
sisted, the  hand  will  be  moved  downward.  The  pull  on  the  hand 
in  this  case  is  called  &  force.  A  force  always  tends  to  produce 
motion  and  it  may,  therefore,  be  defined  as  that  which  produces 
motion  or  tends  to  produce  motion.  If  a  force  is  applied  to  any 
stationary  object,  the  object  will  move  unless  the  force  is  resisted 
or  opposed  by  another  force  equal  in  amount  but  opposite  in 
direction.  If  a  moving  object  is  not  acted  upon  by  any  force, 
or  is  acted  upon  by  forces  which  are  equal  in  amount  but  opposite 
in  direction,  the  object  will  continue  to  move  with  a  uniform 
velocity;  but  if  the  forces  acting  upon  the  moving  object  are  not 
equal  in  amount  and  opposite  in  direction,  the  velocity  will 
increase  as  long  as  such  forces  are  applied. 

In  a  steam  engine,  the  piston  is  acted  upon  by  the  force  of  the 
steam  pressure  which  causes  the  piston  to  move,  and  the  motion 
is  transmitted  to  the  flywheel.  The  velocity  of  the  flywheel  will 
increase  until  the  forces  opposing  its  motion  are  equal  in  amount 
to  those  causing  it  to  move,  after  which  it  will  move  with  uniform 
velocity. 

A  force  is  measured  by  the  number  of  pounds  with  which  it 
pulls;  thus  a  force  of  10  pounds  is  the  downward  force  exerted 
by  a  weight  of  10  pounds,  or  the  force  necessary  to  lift  a  weight  of 
10  pounds. 

A  force  acting  upon  the  crank  pin  or  rim  of  the  flywheel  pro- 
duces what  is  called  a  torque  or  twisting  moment.  A  torque  can- 
not be  measured  in  pounds  because  its  amount  depends  both  upon 
the  force  applied  at  the  circumference  of  the  circular  path  of  the 
force  and  also  upon  the  distance  from  the  center  to  the  point 
at  which  the  force  is  applied.  In  the  case  of  the  engine  crank, 
the  torque  depends  both  upon  the  actual  force  applied  to  the 
crank  pin  and  upon  the  length  of  the  crank  from  the  center 
of  the  shaft  to  the  center  of  the  crank  pin.  The  amount  of  the 
5  63 


54  STEAM  ENGINES 

torque  is  expressed  in  foot-pounds  and  is  equal  to  the  number  of 
pounds  of  force  applied  at  the  circumference  multiplied  by  the 
radius  of  the  circular  path,  in  feet.  If  a  force  of  1000  pounds  is 
applied  to  a  crank  pin  which  is  two  feet  from  the  center  of  a  shaft, 
the  torque  or  twisting  moment  will  be 

1000  X  2  =  2000  foot-pounds 

Work. — When  force  is  applied  to  an  object  in  such  a  manner 
as  to  cause  the  object  to  move,  work  is  performed.  If  the  object 
does  not  move,  no  work  is  performed  no  matter  how  large  the 
applied  force  may  be.  Steam  admitted  behind  the  piston  of  a 
steam  engine  causes  the  piston  to  move,  hence  the  steam  performs 
work  upon  the  piston.  If  the  piston  is  blocked  so  that  it  cannot 
move  and  steam  is  admitted  behind  it,  no  work  will  be  performed 
because,  while  the  steam  pressure  may  be  as  great  as  before, 
the  piston  in  this  case  does  not  move. 

The  amount  of  work  performed  is  always  equal  to  the  force 
applied,  expressed  in  pounds,  multiplied  by  the  distance  through 
which  the  object  moves,  expressed  in  feet.  The  unit  of  work  is 
the  foot-pound  and  is  the  amount  of  work  done  when  a  force 
of  one  pound  moves  through  a  distance  of  one  foot. 

Example. — How  much  work  is  performed  when  a  steam  pressure  of  80 
Ib.  per  sq.  in.  acts  upon  the  piston  of  a  steam  engine  which  is  18  inches 
in  diameter  and  which  moves  it  through  a  distance  of  two  feet? 

Solution. — The  area  of  the  piston  is 

.7854  X  182  =  254.5  sq.  in. 
The  force  acting  on  the  piston  is 

254.5  X  80  =  20,360  Ib. 

The  work  performed  when  this  force  moves  through  a  distance  of  two 
feet  is 

20,360  X  2  =  40,720  ft.-lb. 

Work  and  torque  should  not  be  confused  with  each  other,  even  though 
both  are  expressed  in  foot-pounds.  In  the  case  of  work  there  is  motion; 
in  the  case  of  torque  there  is  no  motion. 

Energy. — Energy  is  the  ability  to  do  work.  Water  falling 
over  a  waterfall  is  able  to  perform  work,  hence  falling  water 
possesses  energy.  Steam  performs  work  in  a  steam  engine, 
hence  steam  possesses  energy.  Anything  which  is  capable  of 


HEAT,  WORK,  AND  PRESSURE  55 

performing  work,  or  producing  motion  by  overcoming  a  force, 
possesses  energy.  „ 

There  are  several  kinds  of  energy  such  as  mechanical  energy  or 
the  energy  of  motion,  electrical  energy,  heat  or  thermal  energy, 
and  chemical  energy;  and  any  of  these  different  kinds -of  energy 
may  be  changed  into  any  other  kind.  In  a  power  plant  the  coal 
which  is  burned  under  the  boilers  contains  chemical  energy, 
due  to  the  various  chemical  substances  of  which  it  is  composed. 
When  the  coal  is  burned,  its  chemical  energy  is  changed  into  heat 
energy.  The  heat  energy  causes  the  water  in  the  boiler  to  form 
steam  and  the  steam  containing  the  heat  energy  is  carried  to  the 
cylinder  of  a  steam  engine.  The  heat  energy  which  the  steam 
contains  is  changed,  in  the  cylinder  of  the  steam  engine,  into  the 
mechanical  energy  of  the  moving  piston.  The  mechanical  energy 
of  the  moving  piston  is  transferred  through  the  connecting  rod 
and  crank  to  the  shaft.  A  direct-connected  generator  on  the 
shaft  changes  the  mechanical  energy  into  electrical  energy  and 
this  electrical  energy  may  be  changed  again  into  mechanical 
energy  by  means  of  a  motor. 

Energy  cannot  be  measured  directly  but  it  may  be  measured 
by  the  effects  which  it  produces.  The  different  kinds  of  energy 
mentioned  above  produce  different  effects,  hence  each  kind  has  a 
different  unit  which  is  based  upon  the  effects  produced  by  this 
kind  of  energy.  The  unit  for  mechanical  energy,  or  the  energy 
of  motion,  is  the  foot-pound  the  same  as  for  the  unit  of  work, 
since  the  effect  of  mechanical  energy  is  to  produce  work.  Any 
object  which  is  capable  of  performing  one  foot-pound  of  work  is 
said  to  contain  one  foot-pound  of  energy. 

Heat. — In  the  study  of  steam  engines  we  are  concerned  prin- 
cipally with  heat  energy,  or  simply  heat,  as  it  is  commonly  called, 
since  the  work  performed  in  the  cylinder  of  a  steam  engine 
comes  from  the  heat  which  is  contained  in  the  steam. 

Both  heat  energy  and  mechanical  energy  are  energies  of  motion 
but  there  is  this  difference  between  them;  that  mechanical  energy 
is  a  motion  of  a  body  taken  as  a  whole  while  heat  energy  is  a 
motion  of  the  particles  of  which  a  body  is  composed,  aside  from 
any  motion  which  the  body  as  a  whole  may  have. 

All  substances  are  composed  of  very  small  particles  called 
molecules,  which  are  so  small  that  they  cannot  be  seen,  even  by 
the  aid  of  the  most  powerful  microscope.  The  molecules  are  in 
constant  motion,  vibrating  back  and  forth,  yet  held  together 


56  STEAM  ENGINES 

by  a  force  of  attraction  which  they  have  for  each  other  and  which 
keeps  them  vibrating  within  certain  limits.  Since  the  molecules 
are  too  small  to  be  seen,  and  vibrate  through  a  very  short  dis- 
tance, they  apparently  cause  no  motion  of  the  body  as  a  whole, 
even  though  they  are  vibrating  back  and  forth  among  themselves 
at  a  very  rapid  rate.  The  energy  of  these  vibrating  molecules 
is  heat  energy,  or  what  is  called  heat. 

If  a  nail  is  placed  on  an  anvil  and  struck  rapidly  with  a  hammer, 
the  nail  becomes  hot  or  its  temperature  is  increased,  and,  if  the 
blows  are  struck  faster,  the  temperature  of  the  nail  becomes 
higher;  that  is,  the  temperature  of  the  nail  depends  upon  the 
rapidity  of  the  hammer  blows.  In  vibrating  back  and  forth, 
the  molecules  of  a  substance  strike  each  other  a  great  many 
blows,,  hence  the  temperature  of  the  substance  depends  upon 
the  rapidity  with  which  the  molecules  vibrate,  in  the  same  way 
that  the  temperature  of  the  nail  in  the  above  example  depends 
upon  the  rapidity  of  the  hammer  blows,  and  the  faster  the  mole- 
cules vibrate  the  higher  the  temperature  of  the  substance  will  be. 
When  heat  is  applied  to  a  substance,  the  molecules  receive 
energy  and  vibrate  faster,  hence,  heating  a  substance  increases 
its  temperature. 

A  substance  may  exist  as  a  solid,  a  liquid,  or  a  gas,  depending 
upon  its  temperature  and  the  amount  of  heat  which  it  contains. 
At  the  lower  temperatures  a  substance  will  be  in  the  form  of  a 
solid;  as  it  is  heated  to  higher  temperatures,  it  changes  into  a 
liquid ;  and  as  it  is  heated  to  still  higher  temperatures,  it  changes 
to  a  gaseous  state.  When  a  solid  substance  is  heated,  its  mole- 
cules vibrate  faster  and  faster  and  the  temperature  of  the  sub- 
stance increases.  The  faster  the  molecules  vibrate  the  longer 
their  path  tends  to  be,  hence,  the  more  nearly  they  come  to  break- 
ing down  the  attraction  which  the  molecules  have  for  each  other 
and  which  preserves  the  shape  of  the  solid  substance.  But  this 
partial  breaking  down  of  the  force  of  attraction  between  mole- 
cules allows  the  substance  to  increase  in  size.  This  explains 
the  expansion  of  substances  when  they  are  heated. 

As  the  solid  substance  is  heated  to  a  higher  temperature,  the 
vibration  of  the  molecules  becomes  so  fast  and  the  blows  which 
they  strike  become  so  hard  that  the  attraction  between  the  mole- 
cules is  partially  broken  down  and  the  substance  takes  a  liquid 
form,  in  which  it  does  not  retain  a  definite  shape,  but  the  mole- 
cules are  free  to  move  with  respect  to  each  other.  Even  in  the 


HEAT,  WORK,  AND  PRESSURE  57 

liquid  form  there  is  a  certain  amount  of  attraction  between  the 
molecules;  enough  to  keep  them  in  one  body. 

As  heat  continues  to  be H  applied  to  the  liquid,  its  molecules 
vibrate  faster,  it  increases  fri  temperature,  and  continues  to 
expand.  Some  of  the  molecules  near  the  surface  of  the  liquid 
vibrate  with  enough  force  to  break  through  the  surface  and  then 
go  into  the  space  above  the  liquid;  some  of  these  returning, 
others  remaining  in  the  outer  space.  This  is  the  effect  known  as 
evaporation.  Finally  the  temperature  of  the  liquid  becomes  so 
high  that  great  numbers  of  the  molecules  pass  into  the  space 
above  the  liquid  and  boiling  begins. 

The  only  difference  between  evaporation  and  boiling  is  that 
evaporation  takes  place  only  at  the  surface  while  in  boiling  the 
liquid  is  changed  into  a  vapor  both  at  the  surface  and  in  the  body 
of  the  liquid,  usually  along  the  surface  through  which  the  heat 
passes  into  the  liquid.  This  part  of  the  vapor  is  formed  in 
bubbles,  which,  being  lighter  than  the  liquid  surrounding  them, 
rise  to  the  surface  and  burst.  In  a  vapor  or  gas  the  attraction 
between  the  molecules  has  been  completely  broken  down  and  they 
are  free  to  move  anywhere  within  the  vessel  which  contains 
them,  hence,  a  gas  always  expands  and  fills  completely  any 
vessel  in  which  it  is  placed.  The  molecules  of  a  gas  are  vibrating 
rapidly  and,  as  there  is  no  attraction  between  them,  they  move 
in  straight  lines  from  one  end  or  side  of  the  vessel  to  the  other. 
In  this  way  they  are  continually  striking  blows  against  the  sides 
of  the  containing  vessel.  Just  as  the  particles  of  water  in  a 
stream  from  a  hose  exert  a  pressure  upon  any  object  which 
the  stream  strikes,  so  the  blows  of  the  molecules  against  all 
sides  of  a  vessel  produce  a  pressure  upon  them  and  cause  the  gas 
to  expand  if  the  vessel  is  enlarged. 

Temperature. — Temperature  should  not  be  confused  with 
heat.  Temperature  is  only  one  of  the  effects  of  heat,  and  is  not 
heat  itself.  Temperature  is  a  measure  of  the  rapidity  of  vibra- 
tion of  the  molecules  and,  therefore,  is  a  measure  of  the  intensity 
of  heat. 

Unit  of  Heat. — Heat  energy  cannot  be  measured  directly  but  is 
measured  by  its  effects.  The  most  common  effect  of  heat  is 
increasing  the  temperature  of  a  substance,  and  as  water  is  one  of 
the  most  common  substances,  the  effect  of  heat  in  raising  the 
temperature  of  water  is  used  in  measuring  quantity  of  heat.  The 
unit  of  heat  is  called  the  British  Thermal  Unit  (abbreviated 


58  STEAM  ENGINES 

B.t.u.)  and  is  taken  as  the  quantity  of  heat  which  is  required 
to  raise  the  temperature  of  one  pound  of  water  from  62°  to  63°  F., 
this  temperature  being  chosen  because  the  -amount  of  heat 
required  to  change  the  temperature  of  one  pound  of  water  one 
degree  varies  slightly  at  different  temperatures.  .  However, 
this  variation  is  so  slight  that  it  may  be  neglected  for  most 
practical  purposes,  and  the  unit  of  heat  taken  as  the  quantity  of 
heat  which  is  required  to  raise  the  temperature  of  one  pound  of 
water  one  degree,  without  reference  to  any  particular  temperature. 

Mechanical  Equivalent  of  Heat.  —  Since  heat  energy  is  capable 
of  performing  work  there  must  be  a  numerical  relation  between 
heat  and  work.  It  has  been  found  by  experiment  that  one 
British  Thermal  Unit  (B.t.u.)  is  equivalent  to  778  foot-pounds  of 
work  and,  therefore,  also  to  778  foot-pounds  of  mechanical  energy. 

Specific  Heat.  —  Experiment  shows  that  different  substances 
require  different  amounts  of  heat  to  raise  their  temperature 
one  degree.  Thus,  one  pound  of  cast  iron  requires  .1189  B.t.u. 
to  raise  its  temperature  one  degree,  while  one  pound  of  lead 
requires  .0305  B.t.u.  to  raise  its  temperature  one  degree.  The 
number  of  heat  units  required  to  raise  the  temperature  of  one 
pound  of  any  substance  one  degree  is  called  the  specific  heat  of 
that  substance.  On  this  basis,  the  specific  heat  of  water,  is  one. 

Power.  —  Power  is  the  rate  of  doing  work.  Work  and  power 
should  not  be  confused.  Work  does  not  take  into  account  the 
length  of  time  required  to  perform  it,  while  power  does.  In  order 
to  raise  a  weight  of  4400  pounds  through  a  distance  of  60  feet, 
the  amount  of  work  required  is  4400  X  60  =  264,000  ft.-lbs., 
and  this  amount  of  work  is  the  same  whether  the  weight  is 
lifted  in  one  minute  or  in  one  hour,  but  the  power  required  to 
raise  the  weight  will  be  greater  for  the  shorter  time  in  which  the 
weight  is  raised  than  for  the  longer  time. 

The  unit  of  power  adopted  in  engineering  work  is  called  the 
horsepower  (abbreviated  Hp.)  and  is  the  performance  of  33,000 
foot-pounds  of  work  in  one  minute.  This  is  equivalent  to  the 
performance  of  550  foot-pounds  of  work  in  one  second,  or  of 
1,980,000  foot-pounds  in  one  hour.  The  264,000  foot-pounds 
of  work  mentioned  in  the  example  above  would  require  : 

264,000 
-   '         =  8  horsepower  if  performed  in  one  minute,  or 


264  000 

i  (\of\  rw\  =  0.133  horsepower  if  performed  in  one  hour. 
iyoUuuu 


HEAT,  WORK,  AND  PRESSURE  59 

Since  778  foot-pounds  of  work  are  equivalent  to  one  British 

33  000 
Thermal  Unit,  one  Horsepower  is  equivalent  to  =       '         =  42.42 

«<  «7o 

B.t.u.  per  minute,  or 

1,980,000 
-— ==£ —  =  2545  B.t.u.  per  hour,  in  round  numbers. 

77o 

Example. — In  a  certain  power  plant  400  pounds  of  coal  are  burned  each 
hour.  The  coal  has  a  heating  value  of  12,000  B.t.u.  per  pound,  of  which 
the  engines  utilize  10  per  cent.  How  much  power  is  developed  by  the 
engines? 

Solution. — Heat  liberated  by  the  burning  coal  per  hour  =  400  X  12,000  = 
4,800,000  B.t.u.     Heat  utilized  by  the  engines  per  hour  =  4,800,000  X  .10 
=  480,000  B.t.u.     Horsepower  equivalent  of  480,000  B.t.u.  per  hour  = 
480,000       1S8fiTTn 
-2545"         88'6  Hp- 

Atmospheric  Pressure. — The  earth  is  surrounded  by  a  body  of 
air  which  exerts  a  pressure  upon  everything  upon  the  surface  of 
the  earth,  and  this  pressure  must  be  taken  into  account  in  nearly 
all  calculations  dealing  with  pressure.  The  pressure  exerted  by 
the  air  is  due  to  its  weight  and  amounts  to  14.7  Ib.  per  sq.  in. 
at  sea  level.  If  the  atmospheric  pressure  is  measured  at  a  point 
above  sea  level,  as  on  a  mountain,  it  will  be  less  than  14.7  Ib.  per 
sq.  in.,  because  the  weight  of  air  above  this  point  is  less.  The 
following  table  shows  the  atmospheric  pressure  at  various  eleva- 
tions above  sea  level.: 

Elevation  above  Atmospheric  pressure 

sea  level  in  feet  Ibs.  per  sq.  in. 

0  14.70 

1,000  14.20 

2,000  13.67 

3,000  13.16 

4,000  12.67 

5,000  12.20 

6,000  11.73 

7,000  11.30 

8,000  10.87 

9,000  10.46 

10,000  10.07 

Besides  varying  with  the  elevation  above  sea  level,  the  atmos- 
pheric pressure  also  varies  slightly  with  the  weather,  but  the 
variation  from  this  cause  is  not  very  great. 

Vacuum. — In  engineering  work  a  vacuum  is  a  space  in  which 
the  pressure  is  less  than  atmospheric  pressure.  An  absolute 


60  STEAM  ENGINES 

vacuum  is  a  space  in  which  there  is  no  pressure.  It  is  almost 
impossible  to  produce  an  absolute  vacuum,  hence  in  most 
engineering  work  we  have  to  deal  with  a  partial  vacuum,  in 
which  there  is  some  pressure  although  not  so  much  as 
atmospheric  pressure. 

Barometer. — The  atmospheric  pressure  may  be  measured  by 
an  instrument  called  a  barometer.  A  simple  form  of  barometer 
may  be  constructed  as  follows:  A  glass  tube  about  32  inches  long, 
closed  at  one  end,  is  completely  filled  with  mercury.  The  finger 
is  then  held  over  the  open  end  of  the  tube  to  prevent 
the  mercury  from  spilling  and  the  tube  is  quickly 
T  inverted  and  the  open  end  placed  in  a  cup  of  mercury, 
as  shown  in  Fig.  46.  If  the  finger  is  then  removed 
I  ,  from  the  end  of  the  tube,  the  mercury  will  sink  in  the 
tube  until  it  stands  at  a  certain  height  H  above  the 
surface  of  the  mercury  in  the  cup,  depending  upon 
the  atmospheric  pressure. 

The  space  above  the  mercury  in  the  tube  is  as  near 
s  a  perfect  vacuum  as  can  be  produced.  The  full  atmos- 
|i  pheric  pressure  acts  upon  the  surface  of  the  mercury 
in  the  cup,  hence  the  mercury  in  the  tube  will  stand 
at  such  height  that  its  weight  will  just  balance  the 
pressure  of  the  atmosphere  on  an  area  equal  to  that  of 
the  cross  section  of  the  tube.  Since  a  cubic  inch 
of  mercury  weighs  .4908  pound,  the  height  of  the 
mercury  in  the  tube,  in  inches,  multiplied  by  .4908 
FIQ.  46.  gives  .^g  atmospheric  pressure  in  pounds  per  square 
inch.  Thus,  if  the  height  H  of  the  mercury  is  29  inches,  the 
atmospheric  pressure  is  29  X  .4908  =  14.23  Ibs.  per  sq.  in. 

Absolute  and  Gage  Pressures. — Gages  which'  are  used  for 
indicating  steam  pressure  are  constructed  so  they  read  the 
amount  of  pressure  above  that  of  the  atmosphere;  that  is, 
they  do  not  read  the  true  pressure  or  pressure  above  an  absolute 
vacuum,  but  instead  have  their  zero  point  at  the  atmospheric 
pressure.  For  this  reason  it  is  necessary  to  add  atmospheric 
pressure  to  that  indicated  by  the  gage  in  order  to  find  the  true 
pressure  or  pressure  above  an  absolute  vacuum.  The  pressure 
indicated  by  a  gage  is  called  gage  pressure  and  pressure  measured 
above  an  absolute  vacuum  is  called  absolute  pressure.  The  abso- 
lute pressure  is  equal  to  the  atmospheric  pressure  plus  the  gage 
pressure. 


HEAT,  WORK,  AND  PRESSURE 


61 


Example. — At  a  certain  place  in  which  the  height  of  the  mercury  in  a 
barometer  stands  at  28.5  inches,  the  steam  gage  on  a  boiler  reads  llOlbs. 
per  sq.  in.  What  is  the  absolute  pressure  of  the  steam  in  the  boiler? 

Solution. — The  atmospheric  preggure  equals 

28.5  X  .4908  =  13.98  (practically  14.0)  Ibs.  per  sq.  in. 
The  absolute  pressure  in  the  boiler  equals 

14  +  110  =  124  Ibs.   per   sq.   in. 

When  the  number  of  pounds  pressure  is  given  without   stating  whether 
it  is  gage  or  absolute  pressure,  it  is  usually  understood  to  be  gage  pressure. 

Measuring  Vacuum. — A  vacuum  is  measured  by  attaching 
it  to  a  mercury  column,  similar  to  a  barometer,  and  reading  the 


FIG.  47. 

number  of  inches  of  mercury  which  it  will  support.  If  the  top 
end  of  the  glass  tube  in  Fig.  46  is  opened  so  the  atmospheric 
pressure  can  act  on  the  surface  of  the  mercury  in  the  tube,  the 
mercury  will  immediately  fall  to  the  same  level  as  that  in  the 
cup.  If,  now,  the  top  of  the  glass  tube  is  attached  to  an  air 
pump,  as  shown  in  Fig.  47,  and  the  pressure  in  the  tube  reduced 
below  that  of  the  atmosphere,  then  the  mercury  will  rise  in  the 


62 


STEAM  ENGINES 


Fio.  48. 


tube  to  a  height  which 
balances  the  difference 
in  pressure  between  the 

CO/VD£K£,Cff 

atmosphere  and  that  in 
the  pump.  For  example,  suppose  the  height 
of  mercury  in  the  tube  attached  to  the 
pump  in  Fig.  47  is  6  inches.  The  reduction 
in  pressure  in  the  tube  then  amounts  to 
16  X  .4908  =  7.85  Ibs.  per  sq.  in 

The  actual  pressure  above  the  mercury 
would  be  the  difference  between  the  atmo- 
spheric pressure  and  7.85  Ib.  per  sq.  in.  If 
the  atmospheric  pressure  were  14.7  Ib.  per 
sq.  in.,  the  actual  pressure  above  the  mer- 
cury would  be 

14.7  -  7.85  =  6.85  Ib.  per  sq.  in. 

It  is  usual  to  read  and  express  the  amount 
of  vacuum  in  terms  of  inches  of  mercury 
thus:  the  amount  of  vacuum  in  the  above 
example  would  be  called  "16  inches  of 
vacuum." 

A  mercury  gage  for  measuring  vacuum 
may  be  constructed  as  follows:  A  glass 
tube  about  68  inches  long  is  bent  into  a 
U-shape  as  shown  in  Fig.48,and  is  attached 
to  a  board  with  a  scale  divided  into  inches 
and  tenths  of  an  inch  placed  between  the 
legs  of  the  U-tube.  Mercury  is  poured  into 
the  glass  tube  until  the  legs  are  about  half 
full,  and  the  instrument  is  then  ready  to 
use.  As  long  as  atmospheric  pressure  acts 
upon  the  surface  of  the  mercury  in  both 
legs  of  the  U-tube,  the  mercury  in  these 
legs  will  stand  at  the  same  height,  but  if 
one  leg  is  attached  to  a  vessel  in  which 
there  is  a  vacuum,  as,  for  example,  the 
condenser  of  a  steam  engine,  then  the  mer- 
cury will  rise  in  this  branch  and  fall  an 
equal  amount  in  the  other  branch.  The 


HEAT,  WORK,  AND  PRESSURE  63 

difference  in  height  of  the  two  columns  of  mercury  will  then 
represent  the  difference  in  pressure  between  the  atmosphere  and 
that  in  the  condenser.  For  example,  suppose  the  difference  in 
height  of  the  two  columns  of  mercury  is  16  inches.  The  re- 
duction in  pressure  in  the  leg  attached  to  the  condenser  is 

16  X  .4908  =  7.85  Ibs.  per  sq.  in.  below  that  of  the 

atmosphere.  If  the  pressure  of  the  atmosphere  is  14.7  Ibs.  per 
sq.  in.,  the  actual  pressure  in  the  condenser  will  be 

14.7  -  7.85  =  6.85  Ibs.  per  sq.  in. 

The  amount  of  vacuum  in  this  example  would  be  called  "16 
inches  of  vacuum." 

Another  form  of  vacuum  gage,  and  the  one  most  commonly 
used,  is  constructed  like  a  pressure  gage,  except  it  reads  pressures 
below  atmosphere  instead  of  above,  and  instead  of  reading  in 
pounds  per  square  inch  it  reads  in  inches  of  mercury;  its  readings, 
therefore,  have  the  same  meaning  as  those  of  the  two  vacuum 
measuring  devices  previously  described. 

It  should  be  noted  that  the  same  amount  of  vacuum  as  indi- 
cated by  a  vacuum  gage  does  not  always  mean  the  same  thing. 
Thus,  in  New  York,  which  is  at  sea  level,  a  vacuum  gage  on  a 
condenser  which  reads  22  inches  of  vacuum  shows  that  the  abso- 
lute pressure  in  the  condenser  is 

14.7  -  (22  X  .4908)  =  4.1  Ibs.  per  sq.  in., 

while  in  Butte,  Montana,  which  is  approximately  5000  feet  above 
sea  level  and  where  the  atmospheric  pressure  is  about  12.2  Ibs. 
per  sq.  in.,  a  vacuum  of  22  inches  would  show  that  the  absolute 
pressure  in  the  condenser  is  only 

12.2  -  (22  X  .4908)  =  1.4  Ib.  per  sq.  in. 


CHAPTER  V 
PROPERTIES  OF  STEAM 

Formation  of  Steam. — Suppose  we  have  an  open  pan  contain- 
ing water  at  a  temperature  of  32°  and  that  the  atmospheric 
pressure  is  14.7  Ib.  per  sq.  in.  If  the  pan  is  placed  over  a  fire, 
the  temperature  of  the  water  begins  to  rise  as  soon  as  the  water 
absorbs  heat.  The  temperature  of  the  water  will  continue  to 
increase  until  it  reaches  212°  when  small  bubbles  of  steam  will 
begin  to  form  at  the  bottom  of  the  pan  and  rise  to  the  surface 
of  the  water  and  burst,  liberating  the  steam  which  they  contain. 
It  will  "be  noted  that  the  temperature  of  the  water  does  not  rise 
above  212°  even  though  we  continue  to  apply  heat  to  it.  If  we 
apply  heat  faster,  the  boiling  occurs  more  rapidly.  If  we  apply 
heat  slower,  the  boiling  occurs  slower,  but  the  temperature  remains 
constant  at  212°.  If  we  place  thermometers  in  both  the  water 
and  the  steam  rising  from  the  water,  both  of  them  will  indicate 
212°,  showing  that  the  temperature  of  the  steam  is  the  same  as 
that  of  the  water  with  which  it  is  in  contact.  The  temperature 
of  the  steam  cannot  be  raised  above  that  of  the  boiling  water 
as  long  as  it  remains  in  contact  with  the  boiling  water,  since  any 
attempt  to  do  so  will  simply  result  in  boiling  the  water  faster. 
The  steam  may,  however,  be  collected  and  removed  from  the 
presence  of  water  and  then  heated  to  a  higher  temperature. 

If  we  should  measure  the  amount  of  heat  supplied  to  the  water 
while  its  temperature  was  increasing  from  32°  to  212°,  we  would 
find  that  180  B.t.u.  had  been  supplied  for  each  pound  of  water 
in  the  pan.  If  we  should  measure  the  amount  of  heat  supplied 
to  the  water  after  it  had  reached  212°,  we  would  find  that  970.4 
B.t.u.  had  been  supplied  for  each  pound  of  steam  formed.  We 
would  also  find,  upon  measurement,  that  while  one  pound  of  the 
water  occupies  a  volume  of  only  .0155  cubic  feet,  the  volume  of 
the  steam  formed  from  it  occupies  a  volume  of  26.79  cubic  feet 
or  about  1700  times  the  volume  of  the  water  from  which  it  was 
formed.  The  large  quantity  of  heat  absorbed  by  the  water  after 
it  has  reached  the  boiling  temperature  is  utilized  in  breaking 

64 


PROPERTIES  OF  STEAM  65 

down  the  attraction  of  the  molecules  for  one  another  and  in 
increasing  the  volume  of  the  steam  from  that  of  the  water  to  that 
of  the  steam,  this  increase  of  volume  taking  place  against  the 
pressure  of  the  water  and  the  pressure  on  the  surface  of  the  water, 
which,  in  this  case,  is  the  atmospheric  pressure  of  14.7  Ib.  per 
sq.  in. 

If  the  water,  in  the  above  example,  had  been  heated  in  a  closed 
vessel  under  a  pressure  greater  than  14.7  Ib.  per  sq.  in.,  the  tem- 
perature at  which  boiling  occurred  would  have  been  greater 
than  212°,  and  if  the  pressure  in  the  vessel  had  been  less  than 
14.7  Ib.  per  sq.  in.,  the  temperature  at  which  boiling  occurred 
would  have  been  less  than  212°,  showing  that  the  temperature 
at  which  water  boils  depends  upon  the  pressure  acting  upon  the 
surface  of  the  water. 

In  the  case  of  the  water  heated  in  an  open  pan,  mentioned 
above,  it  is  to  be  noted  that  the  total  amount  of  heat  required 
to  form  one  pound  of  steam  may  be  divided  into  two  distinct 
parts;  first  the  amount  of  heat  absorbed  by  the  water  in  raising 
its  temperature  from  32°  to  the  boiling  temperature,  and,  second, 
the  amount  of  heat  required  to  change  the  water  into  steam  after 
it  has  reached  the  boiling  temperature.  The  first  of  these  is 
called  the  "heat  of  the  liquid"  or  sometimes  the  "sensible  heat 
of  the  steam,"  because  this  part  of  the  heat  is  sensible  to  the  touch 
or  affects  a  thermometer.  The  second  part  of  the  heat  mentioned 
above  is  called  the  "  latent  heat"  or  the  "latent  heat  of  evaporation," 
because  it  is  not  sensible  heat  but  is  latent.  In  the  case  of  water 
heated  under  a  pressure  of  14.7  Ib.  per  sq.  in.  the  heat  of  the 
liquid  amounts  to  180  B.t.u.  and  the  latent  heat  to  970.4 
B.t.u.  The  sum  of  these  two  quantities  or  180  +  970.4  = 
1150.4  B.t.u.  is  the  total  quantity  of  heat  supplied  in  forming 
one  pound  of  steam  from  one  pound  of  water  at  an  initial  temper- 
ature of  32°.  The  name  of  "total  heat"  is  given  to  this  quantity. 

Steam  Tables. — The  boiling  temperature,  heat  of  the  liquid, 
latent  heat,  total  heat,  and  volume  of  steam  formed  at  various 
pressures  have  been  found  from  experiment,  or  calculated,  and 
this  information  is  placed  in  tables,  called  steam  tables,  one  of 
these  tables  being  found  at  the  end  of  this  chapter.  It  will  be 
noted  that  this  table  is  headed  "  Properties  of  Saturated  Steam." 
By  saturated  steam  is  meant  steam  which  has  the  same  temperature 
at  which  it  was  formed.  Steam  in  contact  with  the  water  from 
which  it  was  formed  is  saturated  steam.  The  various  quantities 


66  STEAM  ENGINES 

given  in  the  steam  table  are  for  one  pound  weight  of  dry  steam, 
hence  to  find  these  quantities  for  any  other  weight  than  one 
pound,  it  is  necessary  to  multiply  the  values  given  in  the  table, 
except  those  of  temperature  and  weight  of  1  cu.  ft.  of  steam,  by 
the  actual  weight  of  the  steam. 

The  properties  of  steam  depend  on  its  absolute  pressure.  The 
pressure  offers  a  certain  amount  of  resistance  to  the  expansion  of 
the  water  into  steam  and  it  is  the  amount  of  this  resistance  which 
determines  the  temperature  of  evaporation  and  other  quantities. 
Consequently,  the  absolute  pressure  is  the  first  item  given  in  the 
table. 

For  convenience,  the  corresponding  gage  pressures  are  given 
in  the  next  column,  assuming  an  atmospheric  pressure  of  14.7 
Ib.  per  sq.  in.  In  'case  the  barometer  shows  an  atmospheric 
pressure  very  different  from  this,  it  is  best  to  add  the  actual 
atmospheric  pressure  to  the  gage  pressure  and  thus  get  the  abso- 
lute pressure,  which  should  then  be  used  for  finding  the  properties 
of  the  steam.  In  using  properties  of  steam  at  pressures  below 
that  of  the  atmosphere,  it  is  especially  desirable  to  calculate  the 
absolute  pressure  from  barometer  and  vacuum  gage  readings 
rather  than  to  use  the  vacuum  gage  reading  in  the  gage  pressure 
column  of  the  table.  An  example  will  show  what  a  difference 
this  will  make. 

Suppose  a  vacuum  gage  on  a  condenser  shows  a  vacuum  of 
27  inches  and  we  want  to  find  the  temperature  of  the  steam  in  the 
condenser.  Without  knowing  the  barometer  reading  but  assum- 
ing the  atmospheric  pressure  to  be  14.7  Ib.  per  sq.  in.,  we  would 
say  that  the  reduction  in  pressure  in  the  condenser  amounted  to 
27  X  .4908  =  13.25  +  Ib.  per  sq.  in.  and  that  at  a  vacuum  of 
13.25  Ib.  (  —  13.25  Ib.  gage),  which  corresponds  to  an  absolute 
pressure  of  14.7  —  13.25  =  1.45  Ib.  per  sq.  in.,  the  temperature 
of  the  steam  (from  the  steam  table)  is  about  115°. 

Now  suppose  we  first  look  at  a  barometer  and  find  that  it 
stands  at  28  inches.  The  condenser  has  more  vacuum  than  we 
thought  it  had.  The  absolute  pressure  in  the  condenser  is 

(28  -  27)  X  .4908  =  .4908  Ib.  per  sq.  in., 

or  not  quite  .5  Ib.  per  sq.  in.  absolute,  and  we  find  from  the  steam 
table  that  the  temperature  of  the  steam  is  a  little  less  than  80° 
instead  of  being  115°. 

The  third  column  in  the  table  gives  the  temperatures  in  which 


PROPERTIES  OF  STEAM  67 

water  boils  when  under  the  pressures  given  in  the  first  column. 
These  temperatures  are  also  the  temperatures  of  saturated  steam 
under  the  given  pressures,  and  likewise  the  temperatures  at 
which  steam  under  the  given  pressures  will  condense. 

The  total  heat  required  to  form  steam  from  water  which  has  an 
initial  temperature  of  32°  is  found  in  the  fifth  column  of  the  table. 
This  quantity  is  the  sum  of  the  heat  supplied  to  the  water,  and 
the  latent  heat. 

The  heat  of  the  liquid,  or  the  heat  in  the  water  above  32°,  is 
found  in  the  third  column.  This  is  the  amount  of  heat  which 
must  be  supplied  to  the  water  to  raise  its  temperature  from  32° 
to  the  boiling  point.  For  approximate  calculations  the  heat  of  the 
liquid  per  pound  of  steam  may  be  found  by  subtracting  32°  from 
the  boiling  temperature,  since  the  specific  heat  of  water  is  nearly 
1,  but  for  accurate  calculations  the  heat  of  the  liquid  should  be 
obtained  from  the  steam  table.  The  different  results  obtained 
by  these  two  methods  may  be  shown  as  follows:  At  165  Ib.  per 
sq.  in.  absolute  pressure  the  boiling  temperature  is  366°.  This 
would  give,  by  difference  of  temperature, 

366  -  32  =  334  B.t.u. 

for  the  heat  of  the  liquid,  while  its  actual  value,  from  the  steam 
table,  is  338.2,  a  difference  of  over  1  per  cent. 

After  water  is  raised  to  the  boiling  point,  heat  must  be  added 
to  change  it  into  steam.  This  heat  is  called  latent  heat,  and  it 
varies  in  amount,  decreasing  as  the  pressure  of  the  steam  in- 
creases. At  an  absolute  pressure  of  14.7  Ib.  per  sq.  in.,  the  latent 
heat  is  970.4  B.t.u.  per  pound  of  steam,  and  at  an  absolute 
pressure  of  100  Ib.  per  sq.  in.  it  is  888.  B.t.u.  .  The  whole  amount 
of  the  latent  heat  will  be  absorbed  only  when  the  whole  pound 
of  water  has  been  evaporated;  also,  when  one  pound  of  steam  is 
condensed,  the  full  latent  heat  will  be  given  up  by  it.  If  the 
water  is  being  evaporated  at  100  Ib.  per  sq.  in.  absolute  pressure 
and  after  reaching  the  boiling  temperature  only  one-half  of  the 
latent  heat  or 

H  X  888  =  444  B.t.u. 

are  supplied  to  the  water,  then  only  one-half  of  a  pound  of  steam 
will  be  formed,  and  conversely,  if  we  extract  444  B.t.u.  from  a 
quantity  of  steam  at  100  Ib.  per  sq.  in.  absolute  pressure,  only 
one-half  of  a  pound  will  be  condensed. 
All  of  the  quantities  given  in  the  steam  table  are  calculated 


68  STEAM  ENGINES 

from  water  at  32°,  and  in  practical  problems  it  is  generally 
necessary  to  calculate  the  heat  in  steam  above  some  other  tem- 
perature than  32°.  Thus  if  we  wish  to  know  how  much  heat  must 
be  supplied  to  one  pound  of  water  at  170°  in  order  to  turn  it  into 
steam  having  a  pressure  of  150  Ib.  per  sq.  in.  absolute,  we  must 
remember  that  the  water  already  contains 

170  -  32  =  138  B.t.u. 

Now,  since  the  total  heat  of  steam  at  150  Ib.  per  sq.  in.  absolute 
is  1193.4  B.t.u.,  there  will  have  to  be  supplied  only 

1193.4  -  138  =  1055.4  B.  t.  u. 

in  order  to  turn  it  into  steam.  Since  the  heat  of  the  liquid  at 
150  Ib.  per  sq.  in.  absolute  pressure  is  330.2  B.t.u.,  only 

330.2  -  138  =  192.2  B.t.u. 

need  be  supplied  to  the  water  to  bring  it  to  the  boiling  tempera- 
ture, but  the  entire  latent  heat,  863.2  B.t.u.,  must  be  supplied 
in  order  to  evaporate  it  into  steam. 

Interpolation  from  Tables. — Interpolation  refers  to  the  method 
used  to  find  values  between  those  given  in  the  tables,  as  for 
example,  finding  the  latent  heat  at  44J^  Ib.  per  sq.  in.  absolute 
pressure.  The  table  gives  the  latent  heat  for  44  Ib.  and  for  45 
Ib.  but  not  for  44 J^  Ib.,  and  we  interpolate  to  get  the  value  for 
44K  Ib.  which  would  be  halfway  between  929.2  and  928.2  or 
just  929.7.  Suppose  we  wish  to  obtain  the  heat  of  the  liquid 
at  120  Ib.  per  sq.  in.  gage  pressure  (134.7  Ib.  absolute).  The 
table  gives  134  Ib.  and  135  Ib.,  the  corresponding  values  of  the 
heat  of  the  liquid  being  321.1  and  321.7.  For  one  pound  change 
in  pressure,  the  heat  of  the  liquid  changes 

321.7   --  321.1  =  .6  B.t.u. 

Now,  134.7  is  .7  Ib.  more  than  134  or  .3  Ib.  less  than  135.  We 
can,  therefore,  add  .7  of  .6  to  321.1  or  subtract  .3  of  .6  from  321.7. 
Either  way  we  get  321.52  as  the  value  of  the  heat  of  the  liquid 
at  120  Ib.  per  sq.  in.  gage  pressure. 

In  interpolating,  remember  that  the  latent  heat  and  the  volume 
of  one  pound  of  steam  decrease  as  the  pressure  increases  and  that 
all  other  items  in  the  table  increase.  For  most  calculations  it  is 
sufficiently  accurate  to  take  the  nearest  value  given  in  the  table 
without  bothering  to  interpolate. 


PROPERTIES  OF  STEAM  69 

Wet  Steam. — Saturated  steam  may  be  either  wet  or  dry.  If 
wet,  it  has  small  particles  of  water  suspended  in  it,  just  as  in  a 
fog  air  has  particles  of  water  suspended  in  it.  The  water  which 
is  suspended  in  wet  steam  has  not  been  evaporated  into  steam  and 
has  not  received  the  latent  heat.  It  is  in  the  form  of  water  but 
is  at  boiling  temperature  and  it  has  therefore  received  the  entire 
heat  of  the  liquid.  Dry  steam  has  no  moisture  in  it  and  has 
received  both  the  entire  heat  of  the  liquid  and  the  entire  latent 
heat.  The  quantities  given  in  the  steam  tables  are  for  dry 
saturated  steam. 

In  changing  water  into  steam  suppose  that  one  pound  of  water 
is  heated  from  32°  to  the  boiling  temperature.  Up  to  this  point 
the  one  pound  of  water  has  received  the  entire  heat  of  the  liquid. 
When  boiling  commences,  suppose  that  one-half  of  the  pound  of 
water  is  evaporated  into  steam  and  the  other  half  of  the  pound  is 
thrown  up  into  the  steam  in  the  form  of  fine  particles  and  remains 
suspended  there.  The  steam  has  then  received  only  one-half 
or  .50  of  the  latent  heat.  If  three-quarters  of  the  water  had  been 
evaporated  into  the  steam  and  the  other  quarter  was  suspended 
in  the  steam  in  the  form  of  water,  the  steam  would  contain 
three-quarters  or  .75  of  the  latent  heat. 

The  total  heat  above  32°  in  one  pound  of  dry  steam  is  equal 
to  the  sum  of  the  heat  of  the  liquid  and  the  latent  heat,  or  calling 
the  total  heat  H,  the  heat  of  the  liquid  h,  and  the  latent  heat  L, 
then 

H  =  h  +  L 

The  total  heat  in  one  pound  of  wet  steam  is  the  sum  of  the 
entire  heat  of  the  liquid,  h,  and  that  fraction  of  the  latent  heat 
which  has  formed  steam,  or 

H  =  h  +  qL 

in  which  q  is  the  per  cent,  of  the  pound  of  steam  which  has  been 
evaporated.  The  quantity,  q,  is  called  the  quality  of  the  steam. 
Example. — A  pound  of  water  at  32°  is  heated  to  the  boiling  temperature 
at  a  pressure  of  100  Ib.  per  sq.  in.  absolute  and  turned  into  steam  having  a 
quality  of  90  per  cent.,  that  is,  90  per  cent,  of  the  pound  of  water  is  evapo- 
rated and  the  other  10  per  cent,  is  suspended  in  the  steam  in  the  form  of 
water.  How  much  heat  has  been  supplied  to  the  steam? 

Solution. — For  100  Ib.  per  sq.  in.  absolute 

h  =  298.3 
L  =  888.0 


70  STEAM  ENGINES 

and  in  this  case  q  =  .  9 

Therefore  H  =  h  +  qL 

=  298.3  +  .9  X  888.0 
=  298.3  +  799.2 
=  1097.5  B.t.u. 

Observe  that  if  the  steam  had  been  dry  it  would  have  contained 

H  =  h  +  L 

=  298.3  +  888.0 
=  1186.3  B.t.u. 

Steam  usually  contains  from  2  to  10  per  cent,  of  moisture  so  that  its 
quality  is  from  90  to  98  per  cent.  Steam  which  has  a  quality  of  98  per  cent, 
or  more  is  called  "commercially  dry  steam." 

In  case  the  steam  is  formed  from  water  having  a  temperature  higher  than 
32°,  the  heat  which  is  already  in  the  water  above  32°  must  be  subtracted 
from  the  total  heat  of  the  steam.  For  example,  suppose  steam  at  150  Ib. 
per  sq.  in.  absolute  pressure  and  having  a  quality  of  95  per  cent,  is  formed 
from  water  having  a  temperature  of  170°.  The  heat  already  in  the  water 
above  32°  is 

170  -  32  =  138   B.t.u. 

hence,  only  330.2  —  138  =  192.2  B.t.u.  need  be  supplied  to  the  water  per 
pound  in  order  to  bring  it  to  the  boiling  temperature.  Since  the  quality  is 
95  per  cent,  only 

.95  X  863.2  =  820  B.t.u. 

is  absorbed  per  pound  in  evaporating  the  water.  Therefore,  this  steam 
has  received 

192.2  +  820  =  1012.2  B.t.u.  per  pound 


Superheated  Steam. — If  saturated  steam  is  taken  away  from 
the  presence  of  water  and  heated,  its  temperature  may  be  raised 
above  that  at  which  it  was  formed.  Steam  which  has  a  higher 
temperature  than  that  at  which  it  was  formed  is  called  super- 
heated steam.  Since  superheated  steam  has  received  heat  above 
that  required  to  form  it  into  saturated  steam,  it  contains  more 
heat  per  pound  than  saturated  steam. 

The  total  heat  above  32°  contained  in  a  pound  of  superheated 
steam  may  be  found  by  adding  to  the  total  heat  of  saturated 
steam  for  the  same  pressure,  as  found  in  the  steam  table,  the 
number  of  heat  units  required  to  superheat  the  steam,  as  shown 
by  the  following  table: 


PROPERTIES  OF  STEAM 
HEAT  UNITS  REQUIRED  TO  SUPERHEAT  STEAM 


71 


Abso- 
lute 
pres- 
sure 

Degrees   of   superheat 

ft 

10 

20 

40 

60 

80 

100 

130 

160          200 

250 

300 

1 

4.9 

9.6 

18.8 

27.9 

36.9 

46.0 

59.6 

73.2        91.3 

114.0 

136.8 

10 

5.4 

10.4 

20.1 

29.6 

39.0 

48.4 

62.4 

76.3 

94.9 

118.0 

141.2 

15 

5.5 

10.6      20.5 

30.2 

39.7 

49.2 

63.3 

77.4 

96.1 

119.4 

142.9 

20 

5.6 

10.8 

20.9 

30.7 

40.3 

49.9 

64.1 

78.3 

97.1 

120.6 

144.2 

30 

5.7 

11.1 

21.4 

31.4 

41.3 

51.0 

65.5 

79.8 

98.8 

122.6 

146.5 

40 

5.9 

11.3 

21.8 

32.0 

42.0 

51.9 

66.6 

81.1 

100.3 

124.2 

148.3 

50 

6.0 

11.5 

22.2 

32.5 

42.4 

52.6 

67.4 

82.1 

101.4 

125.6 

149.8 

60 

6.0 

11.7 

22.5 

32.9 

43.2 

53.3 

68.2 

82.9 

102.4 

126.7 

151.0 

80 

6.2 

11.9 

22.9 

33:6 

44.0 

54.2 

69.3 

84.2 

103.9 

128.4 

152.9 

100 

6.3 

12.2 

23.3 

34.1 

44.6 

55.0 

70.2 

85.2 

105.1 

129.7 

154.4 

130 

6.4 

12.4 

23.8 

34.7 

45.4 

55.8 

71.3 

86.4 

106.4 

131.2 

156.1 

160 

6.5 

12.6 

24.2 

35.3 

46.0 

56.6 

72.1 

87.4 

107.5 

132.5 

157.5 

200 

6.7 

12.9 

24.7 

35.9 

46.8 

57.4 

73.1 

88.6 

108.9 

134.1 

159.3 

250 

6.9 

13.2 

25.1 

36.5 

47.6 

58.4 

74.3 

89.9 

110.4 

135.9 

161.3 

300 

7.0 

13.5 

25.6 

37.1 

48.3 

59.2 

75.3 

91.0 

111.7 

137.4 

163.0 

The  use  of  this  table  may  be  illustrated  by  the  following  example : 

Example. — Determine  the  number  of  heat  units  contained  in  a  pound  of 
superheated  steam  having  a  pressure  of  130  Ib.  per  sq.  in.  absolute  and  having 
a  temperature  of  447.4°. 

Solution. — By  referring  to  the  steam  table  we  see  that  the  temperature 
of  saturated  steam  at  a  pressure  of  130  Ib.  per  sq.  in.  absolute  is  347.4°  and 
that  its  total  heat  is  1191  B.t.u.  The  degree  of  superheat  is,  therefore, 

447.4  -  347.4  =  100° 

And  the  above  table  shows  that  for  this  degree  of  superheat  and  for  a 
pressure  of  130  Ib.  per  sq.  in.  absolute  the  number  of  heat  units  required 
to  superheat  the  steam  is  55.8.  The  pound  of  superheated  steam  will, 
therefore,  contain 

1191  -f  55.8  =  1246.8  B.t.u. 

Since  superheated  steam  contains  more  heat  than  the  same  weight  of  satu- 
rated steam  it  is  evident  that  the  superheated  steam  is  also  dry. 


72 


STEAM  ENGINES 


PROPERTIES  OF  DRY  SATURATED  STEAM 
from  Marks'  and  Davis'  Steam  Tables 


1 

Absolute 
pressure 
Ib.  per  sq.  in. 

2 

Tempera- 
ture; 
degrees 
Fahren- 
heit 

3 

Heat   of 
the  liquid 
per  pound 
B.t.u. 

4 
Latent 
heat  of 
evapora- 
tion  per 
pound 
B.t.u. 

5 

Total 
heat  per 
pound 
B.t.u. 

6 

Volume  of 
one  pound 
cu.  ft. 

7 

Density  or 
weight  of 
one  cu.  ft. 
Ibs. 

P 

t 

h 

L 

H 

v 

d 

0.0886 

32.0 

0.00 

1073.4 

1073.4 

3294.0 

0.000304 

1 

101.83 

69.8 

1034.6 

1104.4 

333.0 

0.00300 

2 

126.15 

94.0 

1021.0 

1115.0 

173.5 

0.00576 

3 

141.52 

109.4 

1012.3 

U21.6 

118.5 

0.00845 

4 

153.01 

120.9 

1005  .  7 

1126.5 

90.5 

0.01107 

5 

162.28 

130.1 

1000.3 

1130.5 

73.33 

0.01364 

6 

170.06 

137.9 

995.8 

1133.7 

61.89 

0.01616 

7 

176.85 

144.7 

991.8 

1136.5 

53.56 

0.01867 

8 

182.86 

150.8 

988.2 

1139.0 

47.27 

0.02115 

9 

188.27 

156.2 

985.0 

1141.1 

42.36 

0.02361 

10 

193.22 

161.1 

982.0 

1143.1 

38.38 

0.02606 

11 

197.75 

165.7 

979.2 

1144.9 

35.10 

0.02849 

12 

201.96 

169.9 

976.6 

1146.5 

32.36 

0.03090 

13 

205.87 

173.8 

974.2 

1148.0 

30.03 

0.03330 

14 

209.55 

177.5 

971.9 

1149.4 

28.02 

0.03569 

15 

213.0 

181.0 

969.7 

1150.7 

26.27 

0.03806 

16 

216.3 

184.4 

967.6 

1152.0 

24.79 

0.04042 

17 

219.4 

187.5 

965.6 

1153.1 

23.38 

0.04277 

18 

222.4 

190.5 

963.7 

1154.2 

22.16 

0.04512 

19 

225.2 

193.4 

961.8 

1155.2 

21.07 

0.04746 

20 

228.0 

196.1 

960.0 

1156.2 

20.08 

0.04980 

21 

230.6 

198.8 

958.3 

1157.1 

19.18 

0.05213 

22 

233.1 

201.3 

956.7 

1158.0 

18.37 

0.05445 

23 

235.5 

203.8 

955.1 

1158.8 

17.62 

0.05676 

24 

237.8 

206.1 

953.5 

1159.6 

16.93 

0.05907 

25 

240.1 

208.4 

952.0 

1160.4 

16.30 

0.0614 

26 

242.2 

210.6 

950.6 

1161.2 

15.72 

0.0636 

27 

244.4 

212.7 

949.2 

1161.9 

15.18 

0.0659 

28 

246.4 

214.8 

947.8 

1162.6 

14.67 

0.0682 

29 

248.4 

216.8 

946.4 

1163.2 

14.19 

0.0705 

PROPERTIES  OF  STEAM 


73 


PROPERTIES  OF  DRY  SATURATED  STEAM — Continued 


1 

Absolute 
pressure 
Ib.  per  sq.  in. 

P 

2 

Tempera- 
ture; 
degrees 
Fahren- 
heit- 

t 

3 

Heat   of 
the  liquid 
per  pound 
B.t.u. 

h 

4 
Latent 
heat  of 
Evapora- 
tion per 
pound 
B.t.u. 

L 

5 

Total 
heat  per 
pound 
B.t.u. 

H 

6 

Volume  of 
one  pound 
cu.  ft. 

V 

7 

Density  or 
weight  of 
one  cu.  ft. 
Ibs. 

d 

30 

250.3 

218.8 

945.1 

1163.9 

13.74 

0.0728 

31 

252.2 

220.7 

943.8 

1164.5 

13.32 

0.0751 

32 

254.1 

222.6 

942.5 

1165.1 

12.93 

0.0773 

33 

255.8 

224.4 

941.3 

1165.7 

12.57 

0.0795 

34 

257.6 

226.2 

940.1 

1166.3 

12.22 

0.0818 

35 

259.3 

227.9 

938.9 

1166.8 

11.89 

0.0841 

36 

261.0 

229.6 

937.7 

1167.3 

11.58 

0.0863 

37 

262.6 

231.3 

936.6 

1167.8 

11.29 

0.0886 

38 

264.2 

232.9 

935.5 

1168.4 

11.01 

0.0908 

39 

265.8 

234.5 

934.4 

1168.9 

10.74 

0.0931 

40 

267.3 

236.1 

933.3 

1169.4 

10.49 

0.0953 

41 

268.7 

237.6 

932.2 

1169.8 

10.25 

0.0976 

42 

270.2 

239.1 

931.2 

1170.3 

10.02 

0.0998 

43 

271.7 

240.5 

930.2 

1170.7 

9.80 

0.1020 

44 

273.1 

242.0 

929.2 

1171.2 

9.59 

0.1043 

45 

274.5 

243.4 

928.2 

1171.6 

9.39 

0.1065 

46 

275.8 

244.8 

927.2 

1172.0 

9.20 

0.1087 

47 

277.2 

246.1 

926.3 

1172.4 

9.02 

0.1109 

48 

278.5 

247.5 

925.3 

1172.8 

8.84 

0.1131 

49 

279.8 

248.8 

924.4 

1173.2 

8.67 

0.1153 

50 

281.0 

250.1 

923.5 

1173.6 

8.51 

0.1175 

51 

282.3 

251.4 

922.6 

1174.0 

8.35 

0.1197 

52 

283.5 

252.6 

921.7 

1174.3 

8.20 

0.1219 

53 

284.7 

253.9 

920.8 

1174.7 

8.05 

0.1241 

54 

285.9 

255.1 

919.9 

1175.0 

7.91 

0.1263 

55 

287.1 

256.3 

919.0 

1175.4 

7.78 

0.1285 

56 

288.2 

257.5 

918.2' 

1175.7 

7.65 

0.1307 

57 

289.4 

258.7 

917.4 

1176.0 

7.52 

0.1329 

58 

290.5 

259.8 

916.5 

1176.4 

7.40 

0.1350 

59 

291.6 

261.0 

915.7 

1176.7 

7.28 

0.1372 

60 

292.7 

262.1 

914.9 

1177.0 

7.17 

0.1394 

61 

293.8 

263.2 

914.1 

1177.3 

7.06 

0.1416 

62 

294.9 

264.3 

913.3 

1177.6 

6.95 

0.1438 

63 

295.9 

265.4 

912.5 

1177.9 

6.85 

0.1460 

64 

297.0 

266.4 

911.8 

1178.2 

6.75 

0.1482 

74 


STEAM  ENGINES 


PROPERTIES  OF  DRY  SATURATED  STEAM — Continued 


1 

Absolute 
pressure 
Ib.  per  sq.  in. 

P 

2 

Tempera- 
ture; 
degrees 
Fahren- 
heit 

t 

3 

Heat  of 

the  liquid 
per  pound 
B.t.u. 

h 

4 
Latent 
heat  of 
evapora- 
tion  per 
pound 
B.t.u. 

L 

5 

Total 
heat  per 
pound 
B.t.u. 

H 

6 

Volume  of 
one  pound 
cu.  ft. 

V 

7 

Density  or 
weight  of 
one  cu.  ft. 
Ibs. 

d 

65 

298.0 

267.5 

911.0 

1178.5 

6.65 

0.1503 

66 

299.0 

268.5 

910.2 

1178.8 

6.56 

0.1525 

.     "  67 

300.0 

269.6 

909.5 

1179.0 

6.47 

0.1547 

68 

301.0 

270.6 

908.7 

1179.3 

6.38 

0.1569 

69 

302.0 

271.6 

908.0 

1179.6 

6.29 

0.1590 

70 

302.9 

272.6 

907.2 

1179.8 

6.20 

0.1612 

71 

303.9 

273.6 

906.5 

1180.1 

6.12 

0.1634 

72 

304.8 

274.5 

905.8 

1180.4 

6.04 

0.1656 

73 

305.8 

275.5 

905.1 

1180.6 

5.96 

0.1578 

74 

306.7 

276.5 

904.4 

1180.9 

5.89 

0.1699 

••••'    75 

307.6 

277.4 

903.7 

1181.1 

5.81 

0.1721 

76 

308.5 

278.3 

903.0 

1181.4 

5.74 

0.1743 

77 

309.4 

279.3 

902.3 

1181.6 

5.67 

0.1764 

78 

310.3 

280.2 

901.7 

1181.8 

5.60 

0.1786 

79 

311.2 

281.1 

901.0 

1182.1 

5.54 

0.1808 

80 

312.0 

282.0 

900.3 

1182.3 

5.47 

0.1829 

81 

312.9 

282.9 

899.7 

1182.5 

5.41 

0.1851 

82 

313.8 

283.8 

899.0 

1182.8 

5.34 

0.1873 

83 

314.6 

284.6 

898.4 

1183.0 

5.28 

0.1894 

84 

315.4 

285.5 

897.7 

1183.2 

5.22 

0.1915 

85 

316.3 

286.3 

897.1 

1183.4 

5.16 

0.1937 

86 

317.1 

287.2 

896.4 

1183.6 

5.10 

0.1959 

87 

317.9 

288.0 

895.8 

1183.8 

5.05 

0.1980 

88 

318.7 

288.9 

895.2 

1184.0 

5.00 

0.2001 

89 

319.5 

289.7 

894.6 

1184.2 

4.94 

0.2023 

90 

320.3 

290.5 

893.9 

1184.4 

4.89 

0.2044 

91 

321.1 

291.3 

893.3 

1184.6 

4.84 

0.2065 

92 

321.8 

292.1 

892.7 

1184.8 

4.79 

0.2087 

93 

322.6 

292.9 

892.1 

1185.0 

4.74 

0.2109 

94 

323.4 

293.7 

891.5 

1185.2 

4.69 

0.2130 

95 

324.1 

294.5 

890.9 

1185.4 

4.65 

0.2151 

96 

324.9 

295.3 

890.3 

1185.6 

4.60 

0.2172 

97 

325.6 

296.1 

889.7 

1185.8 

4.56 

0.2193 

98 

326.4 

296.8 

889.2 

1186.0 

4.51 

0.2215 

99 

327.1 

297.6 

888.6 

1186.2 

4.47 

0.2237 

PROPERTIES  OF  STEAM 


75 


PROPERTIES  OF  DRY  SATURATED  STEAM — Continued 


1 

Absolute 
pressure 
Ib.  per  sq.  in. 

P 

2 

Tempera- 
ture; 
degrees 
Fahren- 
heit 

t 

3 

Heat  of 
the  liqui£ 
per  pound 
B.t.u. 

h 

4 
Latent 
heat  of 
evapora- 
tion per 
pound 
B.t.u. 
L 

5 

Total 
heat  per 
pound 
B.t.u. 

H 

6 

Volume  of 
one  pound 
cu.  f  . 

V 

7 

Density  or 
weight  of 
one  cu.  ft. 
Ibs. 

d 

100 

327.8 

298.3 

888.0 

1186.3 

4.429 

0.2258 

101 

328.6 

299.1 

887.4 

1186.5 

4.388 

0.2279 

102 

329.3 

299.8 

886.9 

1186.7 

4.347 

0.2300 

103 

330.0 

300.6 

886.3 

1186.9 

4.307 

0.2322 

104 

330.7 

301.3 

885.8 

1187.0 

4.268 

0.2343 

105 

331.4 

302.0 

885.2 

1187.2 

4.230 

0.2365 

106 

332.0 

302.7 

884.7 

1187.4 

4.192 

0.2336 

107 

332.7 

303.4 

884.1 

1187.5 

4.155 

0.2408 

108 

333.4 

304.1 

883.6 

1187.7 

4.118 

0.2429 

109 

334.1 

304.8 

883.0 

1187.9 

4.082 

0.2450 

110 

334.8 

305.5 

882.5 

1188.0 

4.047 

0.2472 

111 

335.4 

306.2 

881.9 

1188.2 

4.012 

0.2593 

112 

336.1 

306.9 

881.4 

1188.4 

3.978 

0.2514 

113 

336.8 

307.6 

880.9 

1188.5 

3.945 

0.2535 

114 

337.4 

308.3 

880.4 

1188.7 

3.912 

0.2556 

115 

338.1 

309.0 

879.8 

1188.8 

3.880 

0.2577 

116 

338.7 

309.6 

879.3 

1189.0 

3.848 

0.2599 

117 

339.4 

310.3 

878.8 

1189.1 

3.817 

0.2620 

118 

340.0 

311.0 

878.3 

1189.3 

3.786 

0.2641 

119 

340.6 

311.6 

877.8 

1189.4 

3.756 

0.2662 

120 

341.3 

312.3 

877.2 

1189.6 

3.726 

0.2683 

121 

341.9 

313.0 

876.7 

1189.7 

3.697 

0.2705 

122 

342.5 

313.6 

876.2 

1189.8 

3.668 

0.2726 

123 

343.2 

314.3 

875.7 

1190.0 

3.639 

0.2748 

124 

343.8 

314.9 

875.2 

1190.1 

3.611 

0.2769 

125 

344.4 

315.5 

874.7 

1190.3. 

3.583 

0.2791 

126 

345.0 

316.2 

874.2 

1190.4 

3.556 

0.2812 

127 

345.6 

316.8 

873.8 

1190.5 

3.530 

0.2833 

128 

346.2 

317.4 

873.3 

1190.7 

3.504 

0.2854 

129 

346.8 

318.0 

872.8 

1190.8 

3.478 

0.2875 

130 

347.4 

318.6 

872.3 

1191.0 

3.452 

0.2897 

131 

348.0 

319.3 

871.8 

1191.1 

3.427 

0.2918 

132 

348.5 

319.9 

871.3 

1191.2 

3.402 

0.2939 

133 

349.1 

320.5 

870.9 

1191.3 

3.378 

0.2960 

134 

349.7 

321.1 

870.4 

1191.5 

3.354 

0.2981 

76 


STEAM  ENGINES 


PROPERTIES  OF  DRY  SATURATED  STEAM — Continued 


1 

Absolute 
pressure 
lb.  per  sq.  in. 

P 

2 

Tempera- 
ture; 
degrees 
Fahren- 
heit 

t 

3 

Heat  of 
the  liquid 
per  pound 
B.t.u. 

h 

Latent 
heat  of 
evapora- 
tion per 
pound 
B.t.u. 

L 

5 

Total 
heat  per 
pound 
B.  .u. 

H 

6 

Volume  of 
one  pound 
cu.  ft. 

v 

7 

Density  or 
weight  of 
one  cu.  ft. 
Ibs. 

d 

135 

350.3 

321.7 

869.9 

1191.6 

3.331 

0.3002 

136 

350.8 

322.3 

869.4 

1191.7 

3.308 

0.3023 

137 

351.4 

322.8 

869.0 

1191.8 

3.285 

0.3044 

138 

352.0 

3^3.4 

868.5 

1192.0 

•  3  .  263 

0.3065 

139 

352.5 

324.0 

868.1 

1192.1 

3.241 

0.3086 

140 

353.1 

324.6 

867.6 

1192.2 

3.219 

0.3107 

141 

353.6 

325.2 

867.2 

1192.3 

3.197 

0.3129 

142 

354.2 

325.8 

866.7 

1192.5 

3.175 

0.3150 

143 

354.7 

326.3 

866.3 

1192.6 

3.154 

0.3171 

144 

355.3 

326.9 

865.8 

1192.7 

3.133 

0.3192 

145 

355.8 

327.4 

865.4 

1192.8 

3.112 

0.3213 

146 

356.3 

328.0 

864.9 

1192.9 

3.092 

0.3234 

147 

356.9 

328.6 

864.5 

1193.0 

3.072 

0.3255 

148 

357.4 

329.1 

864.0 

1193.2 

3.052 

0.3276 

149 

357.9 

329.7 

863.6 

1193.3 

3.033 

0.3297 

150 

358.5 

330.2 

863.2 

1193.4 

3.012 

0.3320 

151 

359.0 

330.8 

862.7 

1193.5 

2.993 

0.3341 

152 

359.5 

331.4 

862.3 

1193.6 

2.974 

0.3362 

153 

360.0 

331.9 

861.8 

1193.7 

2.956 

0.3383 

154 

360.5 

332.4 

861.4 

1193.8 

2.938 

0.3404 

155 

361.0 

332.9 

861.0 

1194.0 

2.920 

0.3425 

156 

361.6 

333.5 

860.6 

1194.1 

2.902 

0.3446 

157 

362.1 

334.0 

860.1 

1194.2 

2.885 

0.3467 

158 

362.6 

334.6 

859.7 

1194.3 

2.868 

0.3488 

159 

363.1 

335.0 

859.3 

1194.4 

2.851 

0.3508 

160 

363.6 

335.6 

858.8 

1194.5 

2.834 

0.3529 

161 

364.1 

336.2 

858.4 

1194.6 

2.818 

0.3549 

162 

364.6 

336.7 

858.0 

1194.7 

2.801 

0.3570 

163 

365.1 

337.2 

857.6 

1194.8 

2.785 

0.3591 

164 

365.6 

337.7 

857.2 

1194.9 

2.769 

0.3612 

165 

366.0 

338.2 

856.8 

1195.0 

2.753 

0.3633 

166 

366.5 

338.7 

856.4 

1195.1 

2.737 

0.3654 

167 

367.0 

339.2 

855.9 

1195.2 

2.72 

0.3675 

168 

367.5 

339.7 

855.5 

1195.3 

2.706 

0.3696 

169 

368.0 

340.2 

855.1 

1195.4 

2.690 

0.3717 

PROPERTIES  OF  STEAM 


77 


PROPERTIES  OF  DRY  SATURATED  STEAM — Continued 


1 

Absolute 
pressure 
Ib.  per  sq.  in. 

P 

2 

Tempera- 
ture; 
degrees  - 
Fahren- 
heit 

t 

3   H 

Heat  of 
the  liquid  " 
per  pound 
B.t.u. 

h 

4 
Latent 
heat  of 
evapora- 
tion per 
pound 
B.t.u. 

L 

5 

Total 
h  at  per 
pound 
B.t.u. 

H 

6 

Volume  of 
one  pound 
cu.  ft. 

V 

7 

Density  or 
weight  of 
one  cu.  ft. 
Ibs. 

d 

170 

368.5 

340.7 

854.7 

1195.4 

2.675 

0.3738 

171 

368.9 

341.2 

854.3 

1195.5 

2.660 

0.3759 

172 

369.4 

341.7 

853.9 

1195.6 

2.645 

0.3780 

173 

369.9 

342.2 

853.5 

1195.7 

2.631 

0.3801 

174 

370.4 

342.7 

853.1 

1195.8 

2.616 

0.3822 

175 

370.8 

343.2 

852.7 

1195.9 

2.602 

0.3843 

176 

371.3 

343.7 

852.3 

1196.0 

2.588 

0.3864 

177 

371.7 

344.2 

851.9 

1196.1 

2.574 

0.3885 

178 

372.2 

344.7 

851.5 

1196.2 

2.560 

0.3906 

179 

372.7 

345.2 

851.2 

1196.3 

2.547 

0.3927 

180 

373.1 

345.6 

850.8 

1196.4 

2.533 

0.3948 

181 

373.6 

346.1 

850.4 

1196.5 

2.520 

0.3969. 

182 

374.0 

346.6 

850.0 

1196.6 

2.507 

0.3989 

183 

374.5 

347.1 

849.6 

1196.7 

2.494 

0.4010 

184 

374.9 

347.6 

849.2 

1196.8 

2.481 

0.4031 

185 

375.4 

348.0 

848.8 

1196.8 

2.468 

0.4052 

186 

375.8 

348.5 

848.4 

1196.9 

2.455 

0.4073 

187 

376.3 

349.0 

848.0 

1197.0 

2.443 

0.4094 

188 

376.7 

349.4 

847.7 

1197.1 

2.430 

0.4115 

189 

377.2 

349.9 

847.3 

1197.2 

2.418 

0.4136 

190 

377.6  • 

350.4 

846.9 

1197.3 

2.406 

0.4157 

191 

378.0 

350.8 

846.5 

1197.3 

2.393 

0.4178 

192 

378.5 

351.3 

846.1 

1197.4 

2.381 

0.4199 

193 

378.9 

351.7 

845.8 

1197.5 

2.369 

0.4220 

194 

379.3 

352.2 

845.4 

1197.6 

2.358 

0.4241 

195 

379.8 

352.7 

845.0 

1197.7 

2.346 

0.4262 

196 

380.2 

353.1 

844.7 

1197.8 

2.335 

0.4283 

197 

380.6 

353.6 

844.3 

1197.8 

2.323 

0.4304 

198 

381.0 

354.0 

843.9 

1197.9 

2.312 

0.4325 

199 

381.4 

354.4 

843.6 

1198.0 

2.301 

0.4346 

200 

381.9 

354.9 

843.2 

1198.1 

2.290 

0.437 

205 

384.0 

357.1 

841.4 

1198.5 

2.237 

0.447 

210 

386.0 

359.2 

839.6 

1198.8 

2.187 

0.457 

215 

388.0 

361.4 

837.9 

1199.2 

2.138 

0.468 

220 

389.9 

363.4 

836.2 

1199.6 

2.091 

0.478 

78 


STEAM  ENGINES 


PROPERTIES  OF  DRY  SATURATED  STEAM — Continued 


1 

2 

3 

4 

5 

6 

7 

Absolute 
pressure 
Ib.  per  sq.  in. 

Tempera- 
ture; 
degrees 
Fahren- 
heit 

Heat  of 
the  liquid 
per  pound 
B.t.u. 

Latent 
heat  of 
evapora- 
tion per 
pound 
B.t.u. 

Total 
heat   per 
pound 
B.t.u. 

Volume  of 
one  pound 
cu.  ft. 

Density  or 
weight  of 
one  cu.  ft. 
Ibs. 

P 

t 

h 

L 

H 

V 

d 

225 

391.9 

365.5 

834.4 

1199.9 

2.046 

0.489 

230 

393.8 

367.5 

832.8 

1200.2 

2.004 

0.499 

235 

395.6 

369.4 

831.1 

1200.6 

1.964 

0.509 

240 

397.4 

371.4 

829.5 

1200.9 

1.924 

0.520 

245 

399.3 

373.3 

827.9 

1201.2 

1.887 

0.530 

250 

401.1 

375.2 

826.3 

1201.5 

1.850 

0.541 

260 

404.5 

378.9 

823.1 

1202.1 

1.782 

0.561 

270 

407.9 

382.5 

820.1 

1202.6 

1.718 

0.582 

280 

411.2 

386.0 

817.1 

1203.1 

1.658 

0.603 

290 

414.4 

389.4 

814.2 

1203.6 

1.602 

0.624 

300 

417.5 

392.7 

811.3 

1204.1 

1.551 

0.645 

CHAPTER  VI 


INDICATORS 

Work  Diagrams. — A  diagram  may  be  drawn,  by  means  of  an 
instrument  called  an  indicator,  which  shows  the  work  performed 
by  the  steam  in  the  cylinder  of  a  steam  engine,  and  from  such  a 
diagram  may  be  calculated  the  horsepower  developed  by  the 
engine. 

The  area  of  any  diagram  is  equal  to  the  product  obtained  by 
multiplying  together  its  two  sides.  For  example,  in  Fig.  49 
the  figure  abed  has  one  side  equal  to  6  feet  and  the  other  side  equal 
to  3  feet,  hence  the  area  of  the  diagram  abed  is  6  X  3  =  18 
sq.  ft.  In  Fig.  50  is  a  similar  diagram,  except  that  one  side  repre- 


FIG.  49.  .• 

sents  distance  in  feet  and  the  other  side  represents  force  in  pounds. 
The  area  of  this  diagram  abed  is  equal  to  the  product  of  the  two 
sides  or  6  X  3  =  18,  but  in  this  case  the  area  represents  foot- 
pounds, since  the  product  of  feet  and  pounds  is  foot-pounds  or 

6  X  3  =  18  foot-pounds. 

Since  foot-pounds  is  the  unit  of  work,  the  area  of  the  diagram 
abed  in  Fig.  50,  represents  work,  or  is  a  work  diagram.     In  a 
8  79         . 


80 


STEAM  ENGINES 


similar  manner,  if  a  diagram  is  drawn  so  that  one  side  represents 
distance  and  the  other  side  represents  force  (or  pressure,  which  is 
a  force)  the  area  of  the  diagram  will  represent  foot-pounds  of 
work.  This  is  the  same  principle  upon  which  an  indicator  draws 
a  work  diagram  for  the  cylinder  of  a  steam  engine.  The  indicator 
is  so  arranged  that  it  draws  on  a  sheet  of  paper  a  diagram  which 
represents  by  its  height  the  pressure  of  the  steam  in  the  cylinder 
and  by  its  length  represents  the  stroke  of  the  piston.  Such  a 
diagram,  with  one  side  representing  distance  and  the  other  side 
representing  pressure,  shows  by  its  area  the  work  being  performed 
in  the  cylinder. 


r 
i 


a 


T-4 


FIG.  50. 

The  Indicator. — An  indicator  must  record  the  pressure  in  the 
cylinder  at  every  part  of  the  stroke.  In  order  to  do  this,  the 
indicator  consists  of  two  parts,  one  of  which  moves  in  a  vertical 
direction,  with  the  steam  pressure  in  the  cylinder  and  the  other 
which  moves  in  a  horizontal  direction  in  unison  with  the  piston. 
The  part  moving  horizontally  in  unison  with  the  piston  carries  a 
sheet  of  paper  and  the  part  moving  vertically  in  unison  with  the 
pressure  carries  a  pencil  point.  When  the  pencil  point  is  brought 
into  contact  with  the  paper,  a  diagram  is  drawn  which  shows  the 
pressure  in  the  cylinder  at  every  part  of  the  stroke. 

One  type  of  indicator,  shown  in  Fig.  51,  is  partly  cut  away 
in  order  to  show  the  inside  construction  of  it.  The  device  for 
measuring  the  steam  pressure  consists  of  a  small  cylinder  5, 
fitted  with  a  piston  8,  the  cylinder  being  attached  to  the  clearance 
space  of  the  engine  cylinder  so  that  the  steam  pressure  in  the 


INDICATORS 


81 


clearance  space  acts  upon  the  under  side  of  the  indicator  piston. 
A  piston  rod  10  is  connected"  to  the  piston  by  means  of  a  ball 
joint  in  order  to  give  flexibility  between  the  rod  and  the  piston. 
The  piston  rod  passes  through  the  cylinder  head  2,  and  its  end  is 
joined  to  the  pencil  arm  16  by  means  of  a  short  link  14.  One  end 
of  the  pencil  arm  is  pivoted  at  18.  The  other  end,  which  moves 
over  a  sheet  of  paper  on  the  drum  24,  carries  a  pencil  point,  and 
is  forced  to  move  in  a  vertical  line  by  the  links  14  and  15.  The 
piston  is  held  down  by  a  finely  adjusted  coil  spring  and  the 
steam  pressure  forces  the  piston  upward  against  the  resistance 


\Q 


FIG.  51. 

of  this  spring.  These  springs  are  interchangeable  and  several 
are  supplied  with  <each  indicator.  The  number  of  the  spring 
indicates  the  number  of  pounds  pressure  per  square  inch  which, 
acting  upon  the  piston,  will  move  the  pencil  point  one  inch;  for 
example,  if  a  No.  60  spring  is  in  the  indicator  it  will  require  a 
pressure  of  60  Ib.  per  sq.  in.  on  the  piston  to  move  the  pencil 
point  one  inch  vertically.  Therefore,  the  diagram  drawn  by  the 
indicator  may  be  measured  and  the  pressure  in  the  engine  cylinder 
at  any  point  of  the  stroke  determined.  An  indicator  spring 
should  always  be  used  with  the  instrument  for  which  it  is  intended 
as  its  scale  depends  upon  the  length  of  the  pencil  arm  and  a  slight 
difference  in  the  length  of  the  arm  makes  considerable  difference 


82  STEAM  ENGINES 

in  the  movement  of  the  pencil,  since  the  movement  of  the  pencil 
point  is  usually  five  times  the  movement  of  the  piston.  The 
pencil  arm,  together  with  its  links,  is  free  to  turn  about  the  axis 
of  the  cylinder  so  the  pencil  point  may  be  pressed  against  the  drum 
or  lifted  away  from  it. 

The  piston  is  ground  to  a  close  fit  with  the  cylinder  in  order 
to  make  a  nearly  steam  tight  fit  and,  at  the  same  time,  not  cause 
excessive  friction.  Any  steam  that  leaks  past  the  piston  escapes 
through  the  hole  A  in  the  cylinder  and  is  thus  prevented  from 
collecting  over  the  piston  and  exerting  a  downward  pressure  upon 
it.  This  piston  has  a  number  of  small  grooves  cut  in  its  edge, 
which  serve  to  hold  lubricating  oil  and  also  aid  in  preventing 
the  leakage  of  steam. 

The  drum  24,  which  carries  the  sheet  of  paper  and  which  moves 
in  unison  with  the  engine  piston,  is  placed  parallel  to  the  indicator 
cylinder  and  is  located  at  the  end  of  the  arm  L,  which  forms  a 
part  of  the  indicator  cylinder.  The  drum  is  clamped  to  the  end 
of  the  arm  L,  by  means  of  the  thumb  nut  39.  A  cord  wrapped 
around  the  base  of  the  drum  in  the  groove  27  serves  to  turn  it 
about  its  vertical  axis.  The  cord  takes  its  motion  from  the 
crosshead,  which  has  the  same  motion  as  the  piston.  The  cord 
is  not  usually  attached  directly  to  the  crosshead  because  the 
stroke  of  the  crosshead  is  greater  than  the  circumference  of  the 
drum  and  would  therefore  turn  it  through  more  than  one  complete 
revolution,  which  would  be  undesirable  on  account  of  the  pencil 
point  striking  the  paper  clips,  40.  The  drum  is  usually  provided 
with  stops  which  prevent  it  from  turning  through  a  complete 
revolution. 

The  circumference  of  the  drum  is  about  5  inches  and  it  is  de- 
sirable to  have  the  indicator  diagram  about  3  to  3J^  inches  long, 
while  the  crosshead  may  have  a  stroke  of  2  or  3  feet,  hence  it  is 
necessary  to  use  some  device  which  copies  the  motion  of  the 
crosshead  on  a  reduced  scale  and  to  attach  the  cord  to  this  device, 
which  is  called  a  reducing  motion.  Various  forms  of  reducing 
motions  will  be  described  later. 

On  the  forward  stroke  of  the  crosshead  the  cord  which  is 
wrapped  around  the  drum  is  pulled  outward,  turning  the  drum 
through  a  part  of  a  revolution.  At  the  same  time  a  coil  spring 
inside  the  drum  is  wound  up.  This  coil  spring  has  one  end 
attached  to  the  drum  and  the  other  end  to  the  stationary 
spindle  28,  hence,  when  the  crosshead  makes  the  return  stroke  the 


INDICATORS  83 

drum  turns  in  the  opposite  direction,  keeping  the  cord  taut  and 
rewinding  it  in  its  groove  27.  * 

All  the  moving  parts  of  an  indicator  are  made  as  light  as 
possible,  to  avoid  iaertia  effects,  or  over-travelling.  This  is 
especially  necessary  with  the  piston  and  connected  parts,  as 
these  move  rapidly,  and  inertia  would  cause  the  pencil  point 
to  move  too  high  on  its  upward  stroke  and- too  low  on  its  down- 
ward stroke.  For  the  same  reason,  it  is  advisable  to  have  a 
rather  stiff  spring  in  the  indicator,  or  one  which  will  give  a  dia- 
gram about  1^2  to  2  inches  high.  The  number  of  the  spring  to 
use  depends-  both  upon  the  boiler  pressure  and  the  speed  of  the 
engine;  thus  with  a  boiler  pressure  of  90  Ib.  per  sq.  in.  a  No.  60 
spring  should  be  used  with  a  high  speed  engine,  as  this  will 
give  a  diagram  with  a  maximum  height  of  9%0  =  1M  inches. 
The  number  of  spring  to  be  used  with  any  pressure  may  be  found 
by  dividing  the  pressure  by  the  desired  height  of  diagram,  but  it 
should  be  remembered  that  a  high  speed  engine  requires  a 
stiff er  spring  than  a  slow  speed  one. 

Example. — What  number  of  indicator  spring  should  be  used  with  a  boiler 
pressure  of  125  Ib.  per  sq.  in.  if  it  is  desired  to  obtain  a  diagram  1^  inches 
high? 

1  2^ 

Solution.—  j||  =  83.3 

As  the  nearest  regular  size  of  spring  is  80,  this  would  be  used.     The  regular 
sizes  of  springs  are  8,  10,  12,  16,  20,  30,  40,  50,  60,  80,  100,  120,  150,  and  180. 

An  indicator  is  attached  to  a  cylinder  by  means  of  a  short 
length  of  pipe  and  a  quick-opening  valve  placed  just  below  it. 
This  valve,  which  is  furnished  with  the  instrument,  is  used  so 
that  steam  pressure  may  be  cut  off  when  the  indicator  is  not  being 
used,  thus  reducing  wear  on  the  working  parts. 

All  engine  cylinders  have  holes  at  each  end  which  are  bored  and 
threaded  for  attaching  indicators,  the  holes  entering  the  clearance 
space  so  they  will  not  be  covered  by  the  piston  at  any  time.  If 
possible,  it  is  best  to  use  two  indicators,  one  attached  to  each 
end  of  the  cylinder,  as  this  makes  it  possible  to  take  diagrams 
from  the  two  ends  of  the  cylinder  at  the  same  time  and  also  per- 
mits shorter  connections  between  the  indicator  and  cylinder.  A 
long  indicator  connection  is  likely  to  cause  a  drop  in  pressure 
and  thus  make  the  indicator  give  a  false  record.  Straightway 
valves  are  used  for  the  same  reason. 


84 


STEAM  ENGINES 


A  single  indicator  is  sometimes  attached  as  shown  in  Fig.  52 
and  used  for  taking  diagrams  from  both  ends  of  the  cylinder,  in 
which  case  both  diagrams  are  drawn  on  the  same  sheet  of  paper. 
The  principal  disadvantage  in  using  a  single  indicator  is  the  time 
required  to  shut  off  steam  from  one  end  of  the  cylinder  and  turn 


FIG.  52. 


it  on  from  the  other  end,  which  does  not  allow  diagrams  to  be 
taken  from  the  two  ends  of  the  cylinder  at  the  same  time.  The 
time  elapsing  between  taking  the  diagrams  may,  however,  be 
greatly  shortened  by  using  a  three-way  valve,  as  shown  in  Fig. 


FIG.  53. 

53,  at  the  indicator,  instead  of  the  straightway  valve  shown  in 
Fig.  52. 

Indicators  made  by  the  various  manufacturers  differ  from  each 
other  in  small  details,  but  most  of  them  have  the  general  form 
illustrated  by  Fig.  51.  An  indicator  having  a  different  form  of 


INDICATORS 


85 


pencil  movement  is  shown  in  Fig.  54.  In  this  indicator  one  of 
the  usual  movable  links  is  replaced  by  a  slot  cut  in  a  plate  G 
and  the  pencil  arm  has  a  small  roller  on  its  side  which  moves  in 
this  slot.  The  slot  is*  for  the  purpose  of  giving  a  perfect  straight 
line  motion  to  the  pencil  point  and  for  securing  a  uniform  pro- 
portion between  the  motions  of  the  pencil  point  and  indicator 
piston,  and  it  is  shaped  to  secure  these  results.  In  this  indicator, 


FIG.  54. 

the  drum  spring  is  a  flat  coil  spring  placed  at  the  base  of  the  drum, 
as  shown  at  M. 

Fig.  55  shows  a  form  of  indicator  having  the  spring  outside  the 
cylinder,  the  piston  rod  being  made  longer  to  hold  it.  When  the 
spring  is  placed  inside  the  cylinder  and  used  with  high  pressure 
steam,  the  high  temperature  to  which  it  is  subjected  is  liable  to 
change  its  stiffness  and  hence  its  scale.  The  outside  spring  ar- 
rangement is  intended  to  overcome  this  disadvantage  as  well  as 
to  simplify  the  operation  of  changing  springs  for  different  pres- 


86 


STEAM  ENGINES 


sures.     The  outside  spring  is  especially  adapted  to  superheated 
steam. 

The  indicator  shown  in  Fig.  55  also  illustrates  another  recent 
improvement  in  indicators,  that  is,  a  drum  with  which  a  number 
of  diagrams  may  be  drawn  on  the  same  paper.  It  is  designed  to 
use  a  roll  of  paper  upon  which  the  indicator  traces  a  series  of 
diagrams  which  will  continue  until  the  roll  is  exhausted,  unless 


FIG.  55. 

interrupted  by  the  operator.  The  roll  of  paper  is  located  within 
an  opening  in  the  shell  of  the  drum,  thence  the  paper  passes 
around  the  outside  of  the  drum  and  inward  to  a  central  cylinder, 
to  which  it  is  attached.  Upon  the  top  of  the  drum  is  a  ratchet 
wheel  which  automatically  unwinds  a  small  length  of  paper  from 
the  spool  and  winds  it  on  the  inner  cylinder,  thus  giving  a  series 
of  diagrams  which  overlap  each  other  slightly,  as  shown  in  Fig. 
56.  These  indicators  are  commonly  used  with  engines  in  which 
the  load  changes  rapidly,  such  as  rolling  mill  engines,  because  they 


INDICATORS 


87 


record  the  changes  in  load  and  also  show  the  action  of  the  engine 

under    such    changes.     The  -other  ordinary  form 

of   indicator    is    suitable   f*or   drawing   a  diagram 

during  only  a  single-  revolution,  and  if  the  pencil 

point  is  held  on  the  paper  for  more  than  a  single 

revolution,  the  diagrams  will  be  drawn  upon  each 

other,  making  it  difficult  to  distinguish  them. 

Reducing  Motions. — The  principal  require- 
ment of  a  reducing  motion  is  that  it  shall  re- 
produce accurately  the  motion  of  the  crosshead 
on  a  small  scale.  Some  reducing  motions  which 
are  very  simple  in  construction  do  not  reproduce 
the  motion  of  the  crosshead  accurately. 

A  form  of  reducing  motion  intended  to  be  at- 
tached directly  to  the  indicator  is  illustrated  in 
Fig.  56a.  This  reducing  motion  consists  of  a 
large  pulley  around  which  is  wrapped  the  cord 
which  connects  with  the  crosshead  of  the  engine. 
This  pulley  drives  a  smaller  pulley  by  means  of 
bevel  gears  which  reduce  the  motion.  The  smaller 
pulley  drives  the  indicator  drum  by  means  of  a 
cord  wrapped  around  the  drum  and  the  smaller 
pulley.  This  reducing  motion  is  supplied  with 
several  different  sizes  of  the  smaller  pulley  which 
adapts  it  to  steam  engines  having  piston  strokes 
ranging  from  14  inches  to  72  inches. 

This  type  of  reducing  motion  will  reproduce 
the  motion  of  the  crosshead  accurately  if  cords 
are  used  which  do  not  stretch  and  if  the  cords 
are  prevented  from  piling  on  top  of  each  other  as 
they  wrap  around  the  pulleys. 

The  reducing  motion  shown  in  Fig.  52  is  one  of 
the  simplest  and  most  common  forms.  It  consists 
of  a  wooden  arm  pivoted  at  the  top  to  a  stand 
which  is  attached  either  to  the  floor  or  to  the  frame 
of  the  engine.  The  lower  end  of  this  arm  has  a 
slot  for  receiving  a  pin  fitted  to  the  center  of  the 
wrist  pin.  The  upper  end  of  the  arm  has  a 
curved  block  fastened  to  it  with  a  groove  in  its 
edge  in  which  is  placed  the  cord  to  the  indicator. 
In  order  to  reproduce  accurately  the  motion  of 


88 


STEAM  ENGINES 


the  crosshead  the  curvature  of  the  block  must  be  such  that  the 
distance  ob  from  the  pivot  to  the  center  of  the  wrist  pin  divided 
by  the  distance  oa  from  the  pivot  to  the  center  of  the  cord  must 


FIG.  56a. 


be   constant   at   all  points   of  the  stroke,    and  also  the  pivot 
must   be   directly   over   the   center   of  the  wrist  pin  when  the 


FIG.  57. 


crosshead  is  at  the  middle  of  its  stroke.     Lost  motion  between 
the  slot  and  the  pin  in  the  wrist  pin  must  also  be  avoided. 
The  top  view  of  the  engine  illustrated  in  Fig.  57,  shows  a 


INDICATORS  89 

pantograph  reducing  motion;  one  end  of  the  pantograph  being 
attached  to  the  crosshead  and  the  other  end  to  a  stand  placed  on 
the  floor.  The  indicator  cord  is  attached  to  the  crossbar  a, 
which  is  attached  to  the  bars  b  and  c.  The  reducing  motion 
may  be  used  with  different  lengths  of  stroke  by  attaching  the 
crossbar  a  at  various  points  along  the  bars  b  and  c,  but  always 
fastening  it  in  corresponding  holes  in  b  and  c.  On  account  of 
the  large  number  of  joints  in  the  pantograph  and  the  probability 
of  lost  motion  in  them,  this  form  of  reducing  motion  should  be 
made  of  steel  with  closely  fitting  joints.  When  so  made  the 
pantograph  reproduces  the  motion  of  the  crosshead  accurately. 
In  attaching  the  cord  to  the  reducing  motion,  it  should  always 


•COMPRESS 
LINE. 


POINT    OF  EXHAUST     CLOSURE  ATMOSW£f?/C 

FIG.  58. 

be  arranged  so  it  will  run  in  a  direction  parallel  to  the  crosshead, 
as  in  Fig.  57,  otherwise  the  motion  which  the  drum  receives  will 
not  be  the  same  as  that  of  the  crosshead.  Also  the  cord  must  be 
attached  to  cross  bar  a  on  the  center  line  of  the  pantograph  or  a 
correct  reduction  of  the  crosshead  movement  will  not  be  obtained. 
Several  holes  should  therefore  be  provided  in  cross  bar  a  for 
the  insertion  of  the  pin  so  that  the  latter  may  be  properly  located 
as  the  bar  a  is  moved  from  one  position  to  another  on  bars  b  and 
c  for  different  lengths  of  stroke. 

Indicator  Diagrams. — In  taking  an  indicator  diagram,  a  paper 
card,  especially  prepared  for  the  purpose,  is  placed  on  the  drum 
and  the  cord  attached  to  the  reducing  motion.  Steam  is  then 
turned  into  the  indicator  and  after  it  is  warmed  up  the  pencil 
point  is  touched  to  the  drum  while  the  engine  is  making  a  single 
revolution.  Steam  is  then  turned  off  the  indicator  and  the  pencil 
point  again  touched  to  the  drum  in  order  to  draw  the  atmospheric 
line.  The  resulting  diagram  will  be  similar  to  that  shown  in  Fig.  58. 


90  STEAM  ENGINES 

Since  the  atmospheric  line  is  drawn  while  only  the  pressure  of 
the  atmosphere  is  acting  upon  the  piston  of  the  indicator,  this 
line  represents  the  pressure  of  the  atmosphere  and  it  serves  as  a 
reference  line  for  other  parts  of  the  diagram.  Gage  pressures 
may  be  measured  from  the  atmospheric  line,  but  if  absolute 
pressures  are  desired,  it  will  be  necessary  to  draw  a  line  of  no 
pressure  parallel  with  the  atmospheric  line  and  at  a  distance  below 
it  equal  to  the  atmospheric  pressure  as  read  on  a  barometer,  and 
drawn  to  the  same  scale  to  which  the  diagram  is  drawn. 

The  indicator  diagram  shows  the  varying  pressure  in  the 
cylinder  for  a  complete  revolution  or  a  forward  and  back  stroke, 
and  anything  which  affects  this  pressure  also  affects  the  shape  of 
the  diagram.  It  also  shows,  by  its  area,  the  work  being  per- 
formed in  the  cylinder.  The  method  of  calculating  the  work  from 
the  diagram  will  be  given  later. 

The  diagram  shown  in  Fig.  58  is  from  only  one  end  of  the  cylin- 
der, the  end  towards  the  left,  but  since  it  shows  all  changes  of 
pressure,  it  gives  a  record  of  a  complete  cycle  of  the  events 
taking  place  in  that  end  of  the  cylinder.  These  events  have 
been  marked  on  the  diagram  for  reference.  The  point  at  which 
steam  is  admitted  to  the  cylinder  is  shown  at  the  left,  slightly 
before  the  piston  reaches  the  end  of  its  back  stroke.  As  soon  as 
steam  is  admitted  the  pressure  in  this  end  of  the  cylinder  rises 
to  the  full  admission  pressure.  By  the  time  this  has  occurred 
the  piston  has  reached  the  end  of  its  return  stroke  and  is  starting 
on  its  forward  stroke.  During  the  first  part  of  the  forward  stroke 
steam  is  being  admitted  behind  the  piston,  hence  the  pressure 
remains  constant  during  this  part  of  the  stroke.  The  part  of 
the  diagram  drawn  while  steam  is  being  admitted  is  called  the 
steam  line.  At  the  end  of  the  steam  line  is  the  point  of  cut-off 
at  which  the  admission  valve  closes.  On  account  of  the  gradual 
closing  of  this  valve,  the  pressure  changes  gradually  and  the  point 
of  cut-off  is  not  sharply  defined.  After  the  admission  valve 
closes,  the  steam  in  the  cylinder  expands  behind  the  advancing 
piston,  as  shown  by  the  expansion  line,  the  pressure  of  the  steam 
gradually  becoming  smaller  as  its  volume  increases.  Just  before 
the  piston  has  completed  its  forward  stroke  the  exhaust  valve 
opens  and  the  pressure  of  the  steam  quickly  drops  while  the  piston 
is  completing  its  stroke.  The  exhaust  valve  remains  open  dur- 
ing the  greater  part  of  the  return  stroke,  and  the  piston  pushes 
the  low  pressure  steam  from  the  cylinder,  giving  the  exhaust  line 


INDICATORS 


91 


on  the  diagram.  As  the  ports  and  exhaust  passages  offer  a  cer- 
tain amount  of  resistance  to  "the  flow  of  the  exhaust  steam,  the 
exhaust  line  will  be  above  Ijaie  atmospheric  line  by  a  distance 
which  represents  a  fe*w  pounds  pressure.  This  pressure  is  called 
the  "  back-pressure "  since  it  acts  against  the  advancing  piston. 


FIG.  59. 

Near  the  end  of  the  exhaust  stroke  the  exhaust  valve  closes, 
giving  the  ''point  of  compression"  or  "exhaust  closure."  After 
the  exhaust  valve  is  closed  the  steam  remaining  in  the  cylinder 
is  compressed  until  the  admission  valve  opens,  thus  completing 
the  cycle  of  events  for  this  end  of  the  cylinder. 

Similar  events  occur  in  the  other  end  of  the  cylinder  but  not  at 
the  same  time.     These  may  be  shown  on  a  separate  diagram 


FIG.  60. 

drawn  with  another  indicator,  or,  if  a  single  indicator  is  used  for 
both  ends  of  the  cylinder,  both  diagrams  will  be  drawn  on  one 
paper  or  "card,"  and  this  will  show  the  relative  positions  of  the 
events.  Such  a  double  diagram  is  illustrated  in  Fig.  59,  which 
shows  that  admission  and  expansion  are  occurring  in  the  head 
end  of  the  cylinder  while  exhaust  is  taking  place  from  the  crank 
end,  and  that  admission  and  expansion  are  occurring  in  the  crank 
end  while  exhaust  is  taking  place  from  the  head  end. 


92  STEAM  ENGINES 

Besides  being  used  to  determine  the  power  developed  by  an 
engine,  the  indicator  diagram  also  shows  whether  or  not  the 
engine  and  indicator  are  adjusted  properly.  Faults  in  the  engine 
adjustment  will  be  considered  in  a  later  chapter.  A  few  of  the 
more  common  indicator  faults  will  be  considered  here.  Fig.  60 
shows  diagrams  taken  by  an  indicator  in  which  the  cord  is  too 
long,  thus  allowing  the  drum  to  stop  before  the  crosshead  has 


FIG.  61. 

completed  its  stroke.  It  will  be  seen  that  the  left-hand  ends  of 
both  diagrams  appear  to  be  cut  off,  the  heel  of  one  diagram  and 
the  toe  of  the  other  being  cut  off  on  the  same  line.  The  same 
fault  may  be  caused  by  the  motion  of  the  crosshead  not  being 
reduced  sufficiently,  but  in  this  case  the  cord  is  liable  to  be  broken. 
Sometimes  the  piston  of  a  new  indicator  will  fit  too  tightly, 
causing  it  to  stick  in  the  cylinder.  The  result  will  show  in  a 


FIG.  62. 

stepped  expansion  line  as  in  Fig.  61.  The  steps  will  usually  be 
more  distinct  near  the  beginning  of  the  expansion  line  where  the 
pressure  is  high.  The  same  fault  may  be  caused  in  an  old  indi- 
cator by  a  gummed  piston  which  has  not  been  cleaned  and 
lubricated. 

If  the  spring  used  in  an  indicator  is  too  weak  for  the  pressure, 
the  diagram  will  not  only  be  too  high,  but  its  steam  line  will  be 


INDICATORS  93 

wavy,  especially  near  the  end,  as  shown  in  Fig.  62.  Such  a  wavy 
line  is  caused  by  the  vibration  of  the  spring  when  high  pressure 
steam  is  first  admitted  to  the  cylinder.  The  remedy  for  this  is, 
of  course,  to  use  a  stiffer  spring. 

Expansion  of  Steam. — Between  the  point  of  cut-off  and  release 
the. weight  of  steam  in  the  cylinder  remains  constant  provided 
there  is  no  leakage  of  steam  either  into  or  out  of  the  cylinder. 
The  steam  that  is  in  the  cylinder  simply  expands,  that  is,  its 
volume  increases  and  its  pressure  falls. 

If  we  should  select  a  number  of  points  along  the  expansion  line 
of  an  indicator  diagram  and  multiply  the  absolute  pressure  at 
each  of  these  points  by  the  volume  of  steam  in  the  cylinder  at 
that  point  we  would  find  that  the  product  of  this  multiplication 
would  be  practically  a  constant  number.  This  being  true  it  is 
evident  that  the  pressure  of  the  steam  falls  at  the  same  rate  that 
its  volume  increases.  When  the  volume  of  the  steam  has  in- 
creased to  twice  the  volume  contained  in  the  cylinder  at  the  point 
of  cut-off,  the  absolute  pressure  of  the  steam  will  be  one-half  of 
what  it  was  at  the  point  of  cut-off.  In  like  manner,  when  the 
steam  has  expanded  so  that  its  volume  is  1.5  times  its  volume  at 

2          1 

cut-off  its  absolute  pressure  will  be  «  or  7-=-  of  what  it  was  at 

o        i.o 

the  point  of  cut-off;  and  when  the  steam  has  expanded  so  that 
its  volume  is  4  times  its  volume  at  cut-off,  its  absolute  pressure 
will  be  y±  of  its  absolute  pressure  at  cut-off.  It  should  be 
observed  that  the  volume  of  steam  which  is  expanding  refers 
to  the  total  volume  of  steam  which  is  in  the  cylinder  when  cut-off 
occurs.  This  volume  includes  not  only  the  volume  of  steam 
taken  into  the  cylinder  at  each  stroke,  which  is  the  same  as  the 
volume  displaced  by  the  piston  from  the  beginning  of  its  stroke 
up  to  the  point  of  cut-off,  but  it  includes  also  the  volume  of 
steam  in  the  clearance  space  when  the  piston  is  at  the  beginning 
of  its  stroke. 

Example. — Cut-off  occurs  at  %  stroke  in  a  10"  X  12"  engine  having  12 
per  cent,  clearance.  What  is  the  total  volume  of  steam  in  the  cylinder  at 
the  beginning  of  expansion? 

Solution. — The  area  of  the  piston  is 

"1^  X  .7854  =  .5454  sq.  ft. 

Since  the  length  of  stroke  is  12  in.  or  1  ft.,  the  piston  displacement  is 
.5454  X  1  =  .5454  cu.  ft. 


94 


STEAM  ENGINES 


and  the  clearance  volume  is 

.5454  X  .12  =  .0654  cu.  ft. 

Since  cut-off  occurs  at  %  stroke  the  volume  of  steam  taken  into  the  cylinder 
at  each  stroke  is 

%  X  .5454  =  .3409  cu.  ft. 

and  the  total  volume  of  steam  at  the  beginning  of  expansion  is 
.3409  +  .0654  =  .4053  cu.  ft. 

The  quantities  calculated  in  the  above  example  are  shown  on 
the  indicator  diagram  illustrated  in  Fig.  63.  In  this  illustration 
the  piston  displacement  is  represented  by  the  length  of  the  dia- 
gram. The  clearance  volume  c  which  in  this  case,  is  12  per  cent, 
of  the  piston  displacement  is  represented  by  the  distance  between 
the  end  of  the  diagram  and  the  line  ao,  which  is  the  line  of  no 


FIG.  63. 

volume.  The  line  ao  is  located  by  making  the  distance  c  equal 
to  the  clearance  volume  to  the  same  scale  that  the  length  of  the 
indicator  card  represents  the  piston  displacement.  For  example, 
suppose  the  indicator  diagram  is  2.5  in.  long,  and  this  2.5  in. 

represents  the  piston  displacement  of  the  above  example  or  .5454 

2  5 
cu.  ft.     In  this  case  a  length  of  ~?TFT  or  4.583  inches  would 

.0404 

represent  1  cu.  ft.  of  volume.  Therefore  the  clearance  volume  c, 
which  is  .0654  cu.  ft.  would  be  represented  by  a  length  of  .0654  X 
4.583  =  .2997  in.  or  practically  .3  in.  That  is,  the  no  volume 
line  ao  would  be  drawn  .3  in.  from  the  end  of  the  indicator 
diagram. 

Ratio  of  Expansion. — The  ratio  of  expansion  is  a  measure  of  the 
number  of  times  the  steam  is  expanded  in  the  cylinder.  For 
example,  if  there  are  two  cubic  feet  of  steam  in  the  cylinder  when 


INDICATORS  95 

cut-off  occurs  (at  the  beginning  of  expansion)  and  this  is  expanded 
to  four  cubic  feet,  its  volume  has  been  increased  two  times,  or  its 
ratio  of  expansion  is  two. 

Since  the  increase*  in  volume  of  steam  during  expansion  is 
practically  in  proportion  to  the  decrease  in  pressure,  the  pressure 
of  the  steam  at  the  end  of  expansion  may  be  calculated,  if  the 
admission  pressure  and  the  ratio  of  expansion  are  known. 

Example. — If  the  admission  pressure  is  60  Ib.  per  sq.  in.  and  the  ratio 
of  expansion  is  4,  what  will  be  the  pressure  in  the  cylinder  at  the  end  of 
expansion? 

Solution. — Pressure  at  end  of  expansion 

=  -j-  =  15  Ib.  per  sq.  in. 

It  may  be  seen  from  the  above  discussion  and  example  that  for 
a  given  admission  pressure  the  final  pressure  will  be  lowest  with 
an  early  cut-off,  or  large  ratio  of  expansion.  Also,  if  cut-off 
occurs  late  in  the  stroke,  the  steam  being  expanded  but  little, 
the  pressure  will  be  high  at  the  end  of  the  stroke  when  the  exhaust 
valve  opens.  In  the  latter  case,  the  pressure  remaining  in  the 
steam  is  wasted,  hence  an  early  cut-off  or  large  ratio  of  expansion 
is  more  desirable  than  a  small  ratio  of  expansion. 

The  number  of  times  which  steam  may  be  expanded  in  a 
cylinder  depends  upon  the  admission  pressure  of  the  steam  as  well 
as  upon  the  point  of  cut-off.  If  this  pressure  is  low  and  the 
steam  is  expanded  a  large  number  of  times,  the  final  pressure 
will  be  carried  below  the  exhaust  pressure,  forming  a  loop  at  the 
toe  of  the  diagram,  and  no  useful  work  will  be  gained  from  the 
last  part  of  the  expansion.  For  example,  if  the  admission  pressure 
is  60  Ib.  per  sq.  in.  and  the  ratio  of  expansion  is  6,  the  final  pres- 
sure of  the  steam  will  be  10  Ib.  per  sq.  in.,  which  is  below  atmos- 
pheric pressure.  If  this  engine  exhausts  into  the  atmosphere, 
the  expansion  below  14.7  Ib.  per  sq.  in.  produces  no  useful  work 
because,  when  the  exhaust  valve  opens,  the  pressure  in  the  cylin- 
der will  rise  to  14.7  Ib.  per  sq.  in.  In  fact,  the  pressure  in  the 
cylinder  at  the  end  of  expansion  should  be  a  few  pounds  above 
the  exhaust  pressure  because,  if  the  steam  is  expanded  completely 
to  the  exhaust  pressure,  the  extra  work  gained  is  not  enough  to 
compensate  for  the  friction  of  the  engine  during  the  last  part  of 
the  stroke ;  hence,  instead  of  there  being  a  gain  from  the  expan- 
sion of  the  last  few  pounds  of  pressure,  there  is  actually  a  loss. 

An  approximate  value  of  the  ratio  of  expansion  may  be  taken 

9 


96 


STEAM  ENGINES 


as  the  reciprocal  of  the  fraction  of  the  piston  stroke  at  which  cut- 
off occurs.  For  example,  if  cut-off  occurs  at  ^  stroke  this 
approximate  value  of  the  ratio  of  expansion  is  4;  if  cut-off  oc- 
curs at  %  stroke  the  approximate  ratio  of  expansion  is  ^  or  1  J£. 
This  method  of  computing  the  ratio  of  expansion  gives  only  an 
approximate  value  because  the  clearance  volume  is  neglected. 

Whether  the  ratio  of  expansion  is  calculated  by  the  exact 
method  or  the  approximate  method  it  is  necessary  to  locate  the 
point  of  cut-off  on  the  indicator  diagram.  As  the  point  of  cut- 
off is  not  sharply  denned  on  the  indicator  diagram  it  is  rather 
difficult  to  locate  the  exact  point  of  cut-off;  but  it  may  be  done 
with  a  fair  degree  of  accuracy  by  locating  the  point  of  cut-off 
at  the  point  where  the  downward  curve  of  the  admission  line 
meets  the  upward  curve  of  the  expansion  line. 


E  c 


H  G 


FIG.  64. 


The  difficulty  of  locating  the  point  of  cut-off  exactly  on  indica- 
tor diagrams  has  led  to  the  use  of  the  commercial  cut-off  in  deter- 
mining the  ratio  of  expansion.  The  commercial  cut-off  is 
located  by  drawing  a  horizontal  line  on  the  diagram  through  the 
maximum  admission  pressure  and  extending  the  expansion  line 
up  to  meet  this  line.  The  intersection  of  these  two  lines  is  the 
commercial  cut-off. 

The  method  of  determining  the  commercial  cut-off  and  from 
it  the  ratio  of  expansion  is  illustrated  in  Figs.  64  and  65.  The 
line  EH  is  first  drawn  so  that  the  length  EC  represents  the 
clearance  volume  to  the  same  scale  that  the  diagram  is  drawn. 
The  line  EA  is  then  drawn  through  the  maximum  admissiom 
pressure  and  parallel  to  the  atmospheric  line.  In  case  the 
admission  line  is  wavy,  as  in  Fig.  65,  the  line  EA  is  drawn  at  the 
average  height  of  the  waves.  From  the  point  D  where  the  ex- 


INDICATORS 


97 


pansion  line  changes  direction  of  curvature  the  expansion  line 
is  extended  upward  to  interaect  the  line  EA  at  the  point  B. 
The  point  B  is  the  point  of  commercial  cut-off.  The  fraction  of 
stroke  up  to  the  commercial  cut-off  is 

BC 
AC 


H  G 


FIG.  65. 


and  the  ratio  of  expansion  is 

AC  +  EC 
BC  +  EC 


AE 
BE 


These  distances  may  be  measured  directly  on  the  diagram  or 
they  may  be  calculated  from  the  piston  displacement  and  clear- 
ance volume,  if  the  point  of  commercial  cut-off  is  known. 


CHAPTER  VII 
INDICATED  AND  BRAKE  HORSEPOWER 

Mean  Effective  Pressure. — The  area  of  the  diagram,  which 
represents  the  work  being  performed  in  the  cylinder,  may  be 
found  by  multiplying  together  its  height  and  length,  having 
proper  regard  for  the  scales  of  pressure  and  stroke,  but  since  the 
diagram  is  of  irregular  shape  its  average  height  must  be  used  in 
this  calculation.  The  average  height  of  an  indicator  diagram  is 
called  its  mean  effective  pressure,  abbreviated  M.E.P.  Multiply- 
ing together  the  M.E.P.  in  pounds  per  square  inch,  the  length  of 
the  stroke  in  feet,  and  the  area  of  the  piston  in  square  inches  will 
give  the  number  of  foot-pounds  of  work  performed  during  the 
time  in  which  the  diagram  was  made,  or  one  revolution.  Multi- 
plying the  above  product  by  the  number  of  revolutions  per 
minute  will  give  the  number  of  foot-pounds  of  work  performed 
per  minute.  This  may  be  expressed  in  a  formula  as  follows: 

W  =  Plan 

in  which      W  =  the  number  of  foot-pounds  of  work  per  minute 
P  =  the  M.E.P.  in  pounds  per  sq.  in. 

I  =  the  length  of  the  stroke  in  feet 

a  =  the  area  of  the  piston  in  sq.  in. 

n  =  the  number  of  revolutions  per  minute  (r.p.m.) 

Example. — A  20"  X  24"  engine  makes  240  r.p.m.  and  the  indicator 
diagrams  show  a  M.E.P.  of  63  Ib.  per  sq.  in.  How  many  foot-pounds  of 
work  is  the  engine  performing  per  minute? 

Solution.— The  length  of  the  stroke  is  24"  =  2  ft. 
The  are  of  the  piston  is 

.7854  X  202  =  314.16  sq.  in. 
Hence  the  work  performed  is 
W  =  Plan 

=  63  X  2  X  314.16  X  240 
=  9,500,198  ft.-lb.  per  min. 

The  mean  effective  pressure  may  be  measured  directly  from  the 
indicator  diagram  by  the  method  of  ordinates  or  by  means  of  an 
instrument  called  a  planimeter.  The  method  of  ordinates  con- 
sists in  dividing  the  diagram  into  a  number  of  parts  and  measur- 

98 


INDICATED  AND  BRAKE  HORSEPOWER 


99 


ing  its  height  at  each  of  these  parts,  taking  into  account  the 
scale  to  which  the  diagram  is  drawn,  that  is,  the  number  of  the 
indicator  spring. 

The  ordinate  method  of  measuring  the  mean  effective  pressure 
is  illustrated  by  Fig.  66.  The  limiting  lines  at  right  and  left  of 
the  diagram  are  first  drawn  perpendicular  to  the  atmospheric 
line,  and  the  space  between  them  divided  into  ten  equal  parts. 
Vertical  lines,  as  shown  at  1,  2,  3,  4,  etc.,  running  through  the 
center  of  these  spaces  are  then  drawn  and  the  length  of  each 


FIG.  66. 

between  the  upper  and  lower  lines  of  the  diagram  is  measured. 
Adding  these  lengths  together,  multiplying  their  sum  by  the 
scale  of  the  indicator  spring,  and  dividing  by  10  will  give  the 
average  pressure  or  M.E.P. 

To  obtain  the  centers  of  the  ten  spaces  previously  mentioned,  a 
convenient  method  is  to  take  an  ordinary  scale  and  place  it  as 
shown  in  Fig.  66  so  that  the  diagonal  length  between  the  limits 
of  the  diagram  will  be  exactly  .5  inches.  Then  at  the  left  of  the 
scale  point  off  at  Y±  inch,  and  from  there  on  every  J^  inch  towards 
the  right  of  the  diagram.  The  last  point  will  be  at  4%  inches. 
From  these  points  draw  vertical  lines  through  the  diagram  per- 
pendicular to  the  atmospheric  line. 


100  'STEAM  ENGINES 

A  convenient  method  of  obtaining  the  combined  lengths  of  the 
ordinates  is  to  take  a  narrow  strip  of  paper  and  mark  on  its  edge 
the  height  of  each  ordinate.  Begin  with  No.  1  and  mark  its 
length  on  the  paper.  Then  place  the  mark  made  for  No.  1  at 
the  end  of  No.  2  and  make  a  mark  on  the  paper  at  the  other  end 
of  No.  2  and  so  on  until  the  combined  length  of  the  10  ordinates 
is  obtained.  Measure  this  combined  length  with  an  ordinary 
inch  scale,  multiply  by  the  scale  of  the  spring,  and  divide  by  10. 
The  result  will  be  the  M.E.P.  for  the  diagram.  For  example, 
suppose  the  combined  length  of  the  10  ordinates  in  Fig.  66  meas- 
ures 7.8  inches  and  the  diagram  was  drawn  with 
a  No.  60  spring.  The  M.E.P.  would  then  be: 

7.8  X  60 
— r-~ =  46.8  Ib.  per  sq.  in. 


In  a  noncondensing  engine  having  insufficient 
load  where  the  steam  is  cut  off  at  a  very  early 
of  the  stroke,  a  diagram  may  be  obtained 
similar  to  the  one  shown 
^  ^  ^  in  Fig.  67.  In  this  case 
|  IjJ^  the  diagram  must  be  treated 
as  two  distinct  parts,  the 
loop  at  the  toe  of  the  dia- 
gram near  the  end  of  expansion  being  treated  as  negative  and 
the  other  part  being  treated  as  positive.  The  whole  diagram  is, 
in  this  case,  divided  into  20  equal  parts.  From  the  combined 
lengths  of  the  positive  ordinates  which  number  from  1  to  8, 
subtract  the  combined  lengths  of  the  negative  ordinates,  which 
number  from  9  to  20,  then  multiply  the  difference  by  the 
scale  of  the  spring  and  divide  by  20,  which  is  the  number  of 
ordinates.  The  result  will  be  the  mean  effective  pressure  for  the 
whole  diagram.  For  example,  if  the  combined  length  of  the 
positive  ordinates  is  4  inches  and  the  combined  length  of  the 
negative  ordinates  1.7  inches,  the  difference  would  be  2.3  inches. 
If  the  scale  of  the  spring  is  60,  the  M.E.P.  would  be 

L— »„ =  6.8  Ib.  per  sq.  in. 

which  is  the  average  pressure  for  the  entire  diagram. 

The  ordinate  method  of  measuring  the  mean  effective  pressure, 
described  above,  gives  only  approximate  results.  The  approxi- 
mation is  closer  the  larger  the  number  of  parts  into  which  the 


INDICATED  AND  BRAKE  HORSEPOWER        101 

diagram  is  divided,  but  10  divisions  give  results  which  are  close 
enough  for  most  practical  purposes.  The  most  accurate  and 
quickest  method  of  measuring  the  mean  effective  pressure  is  by 
means  of  a  planimeter  and  this  method  should  be  used  if  a  plani- 
meter  is  at  hand.  ;  . 

There  are  several  makes  of  planimeters  on  the  market,  alike  in 
principle  but  differing  in  details  of  construction.  The  one  shown 
in  Fig.  68  will  be  found  very  convenient  for  measuring  mean 
effective  pressure  from  indicator  diagrams.  This  instrument 
consists  of  two  arms  A B  and  CD  which  are  pivoted  so  they  may 
move  with  respect  to  each  other.  In  preparing  the  instrument 
for  use  the  two  points  E  and  F  on  the  back  of  the  arm  CD  are 
set  a  distance  apart  equal  to  the  length  of  the  diagram.  This 
adjustment  should  be  made  as  close  as  possible  by  hand  and  a 


FIG.  68. 

final  and  closer  adjustment  made  by  means  of  the  screw  G.  The 
two  arms  of  the  planimeter  are  then  held  at  approximately  90° 
to  each  other,  the  point  H  is  placed  near  the  center  of  the  indi- 
cator diagram,  and  the  point  J  is  pressed  firmly  into  the  board 
to  hold  it  stationary,  the  small  weight  K  being  placed  on  it  for 
the  same  purpose.  The  point  H  is  next  placed  at  one  corner  of 
the  diagram,  preferably  the  upper  left-hand  corner,  and  the  small 
wheel  M  turned  until  its  zero  is  opposite  the  zero  on  the  fixed 
scale.  The  instrument  is  now  in  position  for  measuring  the 
M.E.P.  of  the  diagram.  This  is  done  by  tracing  out  the  diagram 
with  the  point  H,  following  around  the  diagram  in  a  clockwise 
direction.  When  the  point  H  has  been  moved  entirely  around  the 
diagram  and  brought  back  to  its  starting  point,  the  number  on 
the  wheel  opposite  the  zero  point  of  the  fixed  scale  is  read.  This 
number,  when  multiplied  by  the  scale  of  the  spring  and  divided 
by  a  constant  which  is  usually  40,  gives  the  M.E.P.  of  the  diagram 
in  pounds  per  sq.  in.  In  case  there  is  a  loop  in  the  toe  of  the 
diagram,  as  in  Fig.  67  the  point  is  first  carried  down  the  expansion 


102  STEAM  ENGINES 

line  and  then  around  the  loop  in  a  counter-clockwise  direction. 
This  automatically  subtracts  the  average  pressure  of  the  loop 
from  the  average  pressure  of  the  remainder  of  the  diagram. 

Indicated  Horsepower.  —  The  rate  at  which  work  is  performed 
in  the  engine  cylinder,  which  is  calculated  from  the  indicator 
diagram,  is  called  the  indicated  horsepower  abbreviated  I.H.P. 
After  the  M.E.P.  of  the  indicator  card  has  been  measured,  the 
indicated  horsepower  may  be  calculated  by  the  formula: 

Plan 
LH-R  =  33^00 

in  which  I.H.P.  is  the  indicated  horsepower 

P  is  the  M.E.P.  in  pounds  per  sq.  in. 

I  is  the  length  of  stroke  in  feet 

a  is  the  area  of  the  piston  in  sq.  in. 

n  is  the  number  of  revolutions  per  minute 

The  above  formula  gives  the  I.H.P.  from  a  single  indicator  dia- 
gram, which  is  taken  from  but  one  end  of  the  cylinder,  hence, 
to  find  the  total  I.H.P.  for  the  engine,  the  I.H.P.  must  be  calcu- 
lated for  each  end  of  each  cylinder  and  their  sum  taken.  It  should 
be  remembered  that  the  piston  rod  occupies  a  portion  of  the 
area  of  the  piston,  and,  for  accurate  results,  its  area  must  be 
subtracted  from  the  area  of  the  piston  when  calculating  the 
I.H.P.  for  the  crank  end  of  the  cylinder. 

Example.  —  Calculate  the  indicated  horsepower  of  a  20"  X  24"  simple 
engine,  running  at  240  r.p.m.,  the  M.E.P.  for  the  head  end  of  the  cylinder 
being  48  Ibs.  per  sq.  in.  and  for  the  crank  end  49  Ibs.  per  sq.  in.  The 
diameter  of  the  piston  rod  is  2^  inches. 

Solution.  —  The  area  of  the  piston  on  the  head  end  is 

.7854  X  202  =  314.16  sq.   in. 
The  area  of  the  piston  rod  is 

.7854  X  2.52  =  4.91  sq.  in. 
The  area  of  the  piston  on  the  crank  end  is 

314.16  -  4.91  =  309.25  sq.  in. 
The  length  of  stroke  is 


The  indicated  horsepower  for  the  head  end  is 

Plan       48X2X314.16X240 


33,000 
The  indicated  horsepower  for  the  crank  end  is 


T  TT  P        Plan   -  49X2X309.25X240  _ 
Lti.V.  -  - 


INDICATED  AND  BRAKE  HORSEPOWER        103 

The  total  I.H.P.  is 

219.3  +  220.4  =  439.7 

The  indicated  horsepower  may  be.  Calculated  approximately  by  using  the 
average  M.E.P.  for  the  two  ends  of  the  cylinder  and  neglecting  the  area  of 
the  piston  rod.  The  total  indicated  horsepower  is  then  calculated  by  the 
formula: 

THP    -   «Plan 
2  33,000 

Applying  this  formula  to  the  above  example  would  give 

T  H  P    -  2  Plan  -  2  48.5X2X314. 16  X  240 
LH'P-  ~2  3^000  ~2~  33,000 

The  result  is  3.5  horsepower  larger  than  the  result  obtained  by  the  other 
method  of  calculation. 

Engine  Constant. — In  the  formula  for  indicated  horsepower 
it  will  be  noted  that,  for  any  particular  engine,  the  part  of  the 
formula 

la 
33,000 

is  a  constant  quantity.  This  quantity  is  called  the  engine 
constant.  Since  the  net  area  of  the  piston  will  be  different  on  the 
head  and  crank  ends,  an  engine  will  have  a  head  end  engine 
constant  and  a  crank  end  engine  constant.  When  the  engine 
constant  has  once  been  .calculated,  the  horsepower  may  be 
found  at  any  time  by  observing  the  M.E.P.  and  speed  and  multi- 
plying these  quantities  by  the  engine  constant. 

Brake  Horsepower. — The  indicated  horsepower  is  the  power 
developed  in  the  cylinder  of  an  engine.  This  power  is  trans- 
mitted through  the  piston  rod,  crosshead,  connecting  rod,  crank 
and  main  shaft  to  the  flywheel  and  a  portion  of  it  is  lost  by  friction 
in  the  various  bearings  of  the  engine.  Hence  the  amount  of 
power  delivered  at  the  flywheel  will  be  smaller  than  that  developed 
in  the  cylinder.  The  power  delivered  to  the  flywheel  is  called 
the  brake  horsepower,  abbreviated  B.H.P.  It  may  be  measured 
by  means  of  a  device  called  a  friction  brake,  hence  the  name  brake 
horsepower. 

A  friction  brake  usually  consists  of  a  band  which  is  clamped 
on  the  face  of  the  flywheel  and  which  may  be  tightened  so  as  to 
produce  more  or  less  friction  between  it  and  the  flywheel.  The 
power  of  the  engine  is  expended  in  overcoming  the  friction  of  the 
brake,  which  is  arranged  in  such  way  that  the  pull  of  the  engine 
upon  the  brake  may  be  measured.  The  brake  horsepower  is 
10 


104 


STEAM  ENGINES 


calculated  from  the  pull. and  speed  of  the  engine  and  the  dimen- 
sions of  the  brake. 

A  common  form  of  friction  brake,  called  a  Prony  brake,  is 
shown  in  Fig.  69.  This  brake  consists  of  a  wooden  beam  C  and  a 
band  B  made  of  a  number  of  hard  wood  blocks  fastened  to  a 
sheet  iron  band  and  passing  around  the  flywheel  A .  The  beam  C 
has  a  steel  knife-edge  fastened  to  its  under  side  near  the  end  and 
resting  on  an  iron  plate  on  top  of  the  stand  E.  The  stand  E 
rests  on  a  platform  scale  so  the  pull  of  the  engine  upon  the  brake 
may  be  weighed.  One  end  of  the  band  containing  the  friction 
blocks  is  fastened  to  the  beam  by  passing  through  it  and  having 


FIG.  69. 


a  nut  on  its  end.  The  other  end  of  the  band  is  held  by  hand 
wheel  D  so  it  may  be  tightened  and  its  friction  adjusted.  The 
edges  of  the  flywheel  form  inwardly  projecting  flanges  so  that  a 
stream  of  water  may  be  run  into  the  flywheel  to  keep  it  cool. 
Preparatory  to  using  the  brake,  the  distance  L  from  the  center 
of  the  brake  to  the  knife-edge  is  measured,  the  stand  E  is  weighed, 
and  the  unbalanced  weight  of  the  brake  about  the  center  line 
FG  is  obtained.  This  may  be  done  by  suspending  the  brake  by  a 
cord  at  the  point  F  while  the  end  of  the  beam  rests  on  a  scale 
and  noting  its  weight.  In  using  the  brake  the  engine  is  brought 
up  to  full  speed  and  the  band  tightened  as  much  as  possible 
without  reducing  the  speed.  The  weight  registered  on  the  scales 
and  the  speed  of  the  engine  are  observed  at  the  same  time. 


INDICATED  AND  BRAKE  HORSEPOWER        105 


The  brake  horsepower  may  now  be  calculated  by  the  formula: 


in  which 


33,000 


B.H.P.  is  the  brake  horsepower 

I  is  the  length  from  center  of  flywheel  to  knife- 

edge  in  feet 

n  is  the  number  of  revolutions  per  minute 
W  is  the  pull  of  the  engine  on  the  brake,  in  pounds 
W  =  weight  registered   on   scales   minus   weight 

of  stand  minus  unbalanced  weight  of  brake 
TT  =  3.1416 


FIG.  70. 

Example. — What  is  the  brake  horsepower  of  a  steam  engine  running  at 
210  r.p.m.  when  fitted  with  a  Prony  brake  which  measures  8  feet  from 
center  of  the  flywheel  to  the  point  of  support  at  the  end  of  the  arm,  the 
scale  reading  742  Ibs.,  the  unbalanced  weight  of  the  brake  being  13  Ibs.,  and 
the  weight  of  the  standard  being  10  Ibs.  ? 


Solution 


Then 
B.H.P.  = 


33,000 


W  =  742  -  13  -  10  =  729  Ib. 
n  =  210  r.p.m. 
I  =  8  ft. 

2X3.1416X8X210  X  719 
33,000 


230  horsepower 


106  STEAM  ENGINES 

Another  form  of  brake  for  measuring  power  is  shown  in  Fig. 
70.  This  form  of  brake  is  called  a  rope  brake  because  the  friction 
which  furnishes  the  load  for  the  engine  is  produced  by  a  rope 
wound  around  the  flywheel.  In  this  brake  the  ends  of  the  rope 
are  attached  to  the  top  crosspiece  C  of  a  wooden  frame  which 
rests  on  a  platform  scales.  The  rope  is  looped  around  the 
flywheel  and  the  middle  attached  to  a  screw  which  passes  through 
the  bottom  crosspiece  C.  This  screw  passes  through  a  hand 
wheel  which  is  used  to  tighten  the  rope  and  thus  regulate  the 
load  on  the  engine.  Instead  of  a  hand  wheel  a  large  nut  may  be 
used  for  this  purpose. 

The  brake  horsepower,  as  measured  with  this  form  of  brake, 
may  be  calculated  from  the  formula: 

T>  TT  P    -=  27rRWn 
33,000 

in  which  R  =  the  radius  of  brake  or  distance  from  center  of 

wheel  to  center  of  rope,  in  feet 

W  =  Pull  of  the  engine  =  weight  indicated  by 
scales  minus  weight  of  wooden  frame,  in 
pounds 

n  =  number  of  revolutions  per  minute 

TT  =  3.1416 

Mechanical  Efficiency. — The  mechanical  efficiency  of  an  engine 
or  its  efficiency  considered  simply  as  a  machine,  is  the  ratio  of  the 
brake  horsepower  to  the  indicated  horsepower  or 

T>     TT    p 

Mechanical  Efficiency  =  T~fTp~ 

This  quantity  is  always  less  than  one,  since  there  is  always  a  loss  of 
power  by  friction  in  the  engine;  that  is,  the  brake  horsepower  is 
always  less  than  the  indicated  horsepower.  The  mechanical 
efficiency  of  steam  engines  varies  from  85  per  cent,  to  95  per  cent. 
The  difference  between  the  indicated  horsepower  and  the 
brake  horsepower  is  the  amount  of  power  required  to  overcome 
the  friction.  This  quantity  is  sometimes  called  the  friction 
horsepower  or 

Friction  horsepower  =  I.H.P.  —  B.H.P. 

The  friction  horsepower  is   the   indicated   horsepower   of   the 
engine  when  it  is  running  without  load. 


CHAPTER  VIII 
ACTION  OF  STEAM  IN  THE  CYLINDER 

Cylinder  Condensation. — It  is  a  well-known  fact  that  the  steam 
engine  is  a  wasteful  machine  for  developing  power  because  it 
turns  into  work  only  a  small  part  of  the  heat  energy  delivered  to 
it.  The  amount  of  work  obtained  from  a  steam  engine  is  often 
only  4  or  5  per  cent,  of  the  amount  of  energy  delivered  to  it,  and 
it  rarely  exceeds  20  per  cent.  This  means  that  from  80  to  96 
per  cent,  of  the  heat  energy  supplied  to  the  steam  engine  is 
wasted  or  at  least  is  not  utilized.  For  example,  suppose  an 
engine  uses  35  pounds  of  dry  saturated  steam  per  hour  for  each 
indicated  horsepower  developed,  the  steam  having  an  absolute 
pressure  of  100  Ib.  per  sq.  in.  The  heat  delivered  to  the  engine 
amounts  to  35  X  1186.3  =  41,520.5  B.t.u.  per  horsepower 
per  hour  and  from  this  amount  of  energy  only  one  horsepower  is 
obtained.  One  horsepower  for  an  hour  is  equivalent  to  2545 
B.t.u.,  therefore,  the  part  of  the  energy  supplied  which  is 
turned  into  work  is  only 

=  .0613  or  6.13  per  cent., 
4 1  jO^U.o 

the  remaining  93.87  per  cent,  being  lost  or  wasted.  A  large  part 
of  this  loss  occurs  through  the  exhaust  but  another  considerable 
part  occurs  through  the  condensation  of  steam  in  the  cylinder. 

Cylinder  condensation  is  caused  by  the  alternate  cooling  and 
heating  of  the  cylinder  walls  as  they  are  alternately  in  contact 
with  high  pressure  steam  (which  has  a  high  temperature)  during 
admission,  and  to  low  pressure  steam  (which  has  a  lower  tempera- 
ture) during  exhaust. 

The  exchanges  of  heat  taking  place  between  the  steam  and 
cylinder  walls  may  best  be  studied  by  considering  the  cycle  of 
events  occurring  in  only  one  end  of  the  cylinder.  Exhaust  occurs 
during  the  greater  part  of  the  return  stroke  of  the  piston  and 
during  this  time  the  cylinder  walls,  face  of  the  piston,  and 
cylinder  head  are  in  contact  with  steam  having  a  comparatively 
n  107 


108  STEAM  ENGINES 

low  temperature,  thus  cooling  these  parts  of  the  engine.  Com- 
pression at  the  end  of  the  return  stroke  raises  somewhat  the 
temperature  of  the  steam  in  the  clearance  space,  but  the  warming 
effect  on  the  cylinder  is  small  because  the  temperature  of  the 
compressed  steam  is  not  so  high  as  the  steam  admitted  from  the 
boiler  and  the  piston  is  near  the  end  of  its  stroke,  exposing  very 
little  of  the  cylinder  walls  to  the  compressed  steam.  Most  of 
the  surface  so  exposed  consists  of  the  face  of  the  piston  and  the 
cylinder  head.  Consequently,  when  the  admission  valve  opens 
and  a  fresh  charge  of  high  pressure  (and  high  temperature) 
steam  rushes  into  the  cylinder  it  meets  comparatively  cool  metal 
surfaces  and  a  part  of  it  is  condensed,  collecting  in  a  thin  layer 
of  water  on  these  surfaces.  As  the  piston  advances  on  its 
forward  stroke  it  uncovers  more  and  more  of  the  chilled  cylinder 
walls  which  condense  still  more  of  the  steam  which  is  being 
admitted  to  the  cylinder,  with  the  result  that,  up  to  the  point  of 
cutoff,  from  30  to  50  per  cent,  of  the  steam  fed  into  the  cylinder 
during  admission  is  condensed,  thus  requiring  that  a  greater 
volume  of  steam  be  supplied  to  the  cylinder  than  if  none  of  it  was 
condensed.  The  condensation  occurring  up  to  the  point  of 
cut-off  is  called  initial  condensation. 

After  cut-off  the  piston  continues  to  advance  and  uncover  more 
of  the  cooled  cylinder  walls.  Hence,  condensation  continues 
after  cut-off  but  at  a  lessening  rate  because,  after  cut-off,  the 
steam  in  the  cylinder  is  expanding  and  its  temperature  falling. 
The  difference  in  temperature  between  the  steam  and  the 
cylinder  walls  is  not  so  great.  Besides  the  condensation  due  to 
contact  between  the  steam  and  cooler  cylinder  walls  there  is 
now  also  a  certain  amount  of  condensation  caused  by  energy  being 
taken  out  of  the  expanding  steam  to  move  the  piston. 

Whatever  steam  is  condensed  during  the  early  part  of  the 
stroke  is  deposited  in  the  form  of  a  thin  film  of  water  on  the 
cylinder  walls,  the  face  of  the  piston,  and  the  cylinder  head.  This 
film  of  water  has  a  temperature  equal  to  that  of  the  steam  from 
which  it  was  formed,  hence  it  is  at  or  very  near  the  boiling  point 
corresponding  to  the  pressure  of  the  steam.  After  cut-off  the 
steam  in  the  cylinder  begins  to  expand  and  its  pressure  falls. 
This  lowers  the  boiling  point  below  the  temperature  of  the  water 
already  deposited  on  the  inside  of  the  cylinder  with  the  result 
that  this  water  begins  to  reevaporate.  As  expansion  proceeds, 
the  boiling  point  is  lowered  and  the  difference  between  the  boiling 


ACTION  OF  STEAM  IN  THE  CYLINDER         109 

point  and  the  temperature  of  the  layer  of  water  becomes  larger, 
therefore  ree vapor ation  proceeds  at  a  faster  and  faster  rate. 
For  this  reason,  a  point  is  reached  soon  after  cut-off  where  the 
reevaporation  balances  the  condensation  and  at  this  point  the 
amount  of  water  in  the  cylinder  is  a  maximum.  From  this 
point  on  reevaporation  occurs  faster  than  condensation  and  the 
amount  of  water  in  the  cylinder  grows  smaller.  When  release 
occurs,  there  is  a  sudden  drop  in  pressure,  accompanied  by  a 
sudden  drop  in  the  boiling  point,  and  the  layer  of  water  on  the 
cylinder  walls  reevaporates  very  fast.  During  exhaust  the 
pressure  remains  low  and  reevaporation  continues  at  a  rapid 
rate,  if  there  is  still  any  water  remaining  in  the  cylinder.  If  the 
initial  condensation  has  not  been  very  great,  however,  the  water 
may  be  all  reevaporated  at  the  beginning  of  exhaust,  and  the 
exhaust  steam  will  then  be  dry. 

It  might  be  thought  that  if  the  water  in  the  cylinder  is  all  re- 
evaporated  no  harm  would  be  done  It  should  be  remembered, 
however,  that  any  water  reevaporated  near  the  end  of  expansion 
is  at  a  lower  pressure  than  when  condensed  and  consequently 
it  cannot  be  expanded  as  much  as  if  it  had  not  been  condensed 
but  had  remained  in  the  form  of  steam  and  expanded  through 
the  whole  range  of  pressure. 

If  the  steam  admitted  to  the  cylinder  already  contains  some 
water,  as  would  be  the  case  if  wet  steam  were  supplied,  the  amount 
of  water  reevaporated  during  expansion  and  exhaust  may  be 
greater  than  the  condensation  during  admission.  This  would 
cause  a  much  larger  quantity  of  heat  to  be  taken  from  the  cylinder 
walls  and  the  chilling  effects  of  reevaporation  to  be  greatly  in- 
creased. It  is  an  advantage  therefore  to  supply  only  perfectly 
dry  steanTtcTan  engine  in  order  to  reduce  the  amount  of  water 
in  the  cylinder.  , 

In  the  above  discussion  of  cylinder  condensation,  the  events 
occurring  in  ony  one  end  of  the  cylinder  have  been  considered. 
If  we  consider  these  events  as  occurring  in  the  head  end,  for 
instance,  then  the  events  occurring  in  the  crank  end  have  some 
influence  upon  the  condensation  and  reevaporation  in  the  head 
end.  During  the  first  part  of  exhaust  from  the  head  end,  this 
end  of  the  cylinder  is  in  contact  with  low  temperature  steam 
and  may  be  further  cooled  by  reevaporation,  but  at  the  same 
time  admission  and  expansion  are  occurring  in  the  crank  end  and 
this  end  of  the  cylinder  is  being  warmed  slightly  by  contact -with 


110  STEAM  ENGINES 

high  temperature  steam.  Admission  and  expansion  in  the 
crank  end,  therefore,  reduces  slightly  the  cooling  of  the  head  end 
and  hence  reduces  the  amount  of  condensation  that  would  occur 
in  the  head  end.  The  reduction  in  initial  condensation  from 
this  cause  will  depend  upon  the  lateness  of  the  cut-off  in  the 
opposite  end  of  the  cylinder,  the  condensation  being  less  for  a 
late  cut-off  than  for  an  early  one  because  a  late  cut-off  exposes 
more  of  the  cylinder  walls  to  high  temperature  steam. 

The  cooling  effects  of  reevaporation  depend  upon  the  reduction 
in  .pressure  of  the  steam  during  expansion,  being  greater  the  more 
fully  the  steam  is  expanded.  The  greatest  amount  of  expansion 
occurs  with  an  early  cut-off,  hence,  an  early  cut-off  increases 
reevaporation.  It  will  thus  be  seen  that  the  loss  of  steam  by  con- 
densation and  the  cooling  of  the  cylinder  by  reevaporation  are 
both  increased  by  an  early  cut-off. 

The  give  and  take  of  heat  between  the  steam  and  cylinder  walls 
does  not  affect  all  of  the  metal  in  the  cylinder  because  the  heat 
transfers  occur  so  rapidly  that  their  effects  do  not  have  time  to 
extend  very  far  into  the  metal.  The  outside  of  the  cylinder 
assumes  a  temperature  between  that  of  the  exhaust  and  the 
admission  steam  and  this  temperature  remains  practically 
constant  while  the  engine  is  running.  The  inner  surfaces  of  the 
cylinder  and  piston,  however,  experience  great  changes  in  tem- 
perature, being  alternately  heated  and  cooled,  but  such  changes 
of  temperature  grow  less  the  greater  the  distance  from  the  inner 
surface  at  which  they  are  measured.  It  is  probable  that  the 
depth  of  metal  thus  affected  does  not  average  more  than  .02 
to  .03  inch,  and  this  depth  is  less  for  high  rates  of  revolution 
than  for  low  rates  because  there  is  not  time,  with  a  high  rate  of 
revolution,  for  the  transfer  of  heat  to  take  place.  Other  things 
being  equal,  the  losses  from  cylinder  condensation  and  re- 
evaporation  are  less  for  high  speed  engines  than  for  low  speed 


It  has  been  pointed  out  in  a  previous  paragraph  that  the 
amount  of  heat  taken  from  the  cylinder  walls  by  reevaporation 
exceeds  the  amount  given  to  the  walls  by  condensation.  In  some 
cases  the  amount  of  heat  taken  from  the  cylinder  walls  may  be 
only  slightly  greater  than  that  given  to  them  and,  in  such  cases, 
it  might  be  thought  that  the  cooling  effect  would  be  very  small. 
In  this  connection,  however,  it  should  be  remembered  that  the 
transfer  of  heat  affects  only  a  thin  layer  of  metal  and  the  weight 


ACTION  OF  STEAM  IN  THE  CYLINDER         111 

of  metal  affected,  therefore,  is  small.  Since  one  B.t.u.  will 
change  the  temperature  of  7K  Ibs.  of  cast  iron  one  degree  or  one 
pound  of  cast  iron  7^  degri^s  it  will  be  seen  that  the  transfer 
of  even  a  small  amount  of  heat  may  produce  comparatively 
great  changes  of  temperature  in  the  inner  surfaces  of  the  cylinder. 

The  effects  of  high  speed  and  late  cut-off  in  reducing  the  losses 
from  cylinder  condensation  and  reevaporation  have  been  noted. 
Other  remedies  that  are  used  for  accomplishing  this  purpose  are : 
the  use  of  superheated  steam;  the  use  of  a  steam  jacket  surround- 
ing the  cylinder;  and  compounding,  or  dividing  the  total  range 
of  expansion  between  two  or  more  cylinders.  The  effects  of 
compounding  will  be  considered  in  a  later  chapter. 

The  benefits  to  be  derived  from  the  use  of  superheated  steam 
come  from  the  prevention  of  initial  condensation.  Superheated 
steam  contains  more  heat  per  pound  than  saturated  steam  at  the 
same  pressure,  and,  before  it  can  be  condensed,  the  extra  heat 
which  it  contains  must  first  be  taken  out  of  it,  thus  reducing  its 
temperature  and  changing  it  into  saturated  steam.  Taking  more 
heat  from  it  will  then  cause  condensation.  When  superheated 
steam  is  supplied  to  an  engine  the  heat  needed  to  warm  the 
cooled  cylinder  walls  may  be  supplied  from  the  extra  store  of 
heat  which  the  steam  contains  and  there  will  be  no  initial  con- 
densation. In  order  to  fully  accomplish  this  purpose,  however, 
the  steam  must  be  superheated  enough  to  secure  dry  steam  at  the 
point  of  cut-off.  If  it  is  not  superheated  to  this  degree  all  the 
excess  heat  will  be  taken 'from  the  steam  before  cut-off  and  it 
will  then  begin  to  condense  and  deposit  moisture  on  the  cylinder 
walls. 

There  is  but  little  advantage  in  using  steam  which  is  super- 
heated to  such  an  extent  that  it  will  still  be  superheated  after 
expansion  commences,  because  reevaporation  begins  at  this 
point  and  also  because  the  condensation  which  occurs  before  cut- 
off causes  more  serious  loss  than  that  which  occurs  after  cut-off, 
and  it  is,  therefore,  of  more  advantage  to  prevent  the  initial 
condensation. 

Since  initial  condensation  is  usually  greatest  in  the  plain  slide 
valve  type  of  engine,  which  employs  a  late  cut-off,  it  is  to  be 
expected  that  more  benefit  may  be  derived  from  the  use  of  super- 
heated steam  in  this  type  of  engine  than  from  those  of  other 
types,  employing  an  earlier  cut-off,  or  those  making  use  of  com- 
pound expansion. 


112  STEAM  ENGINES 

A  steam  jacket  consists  of  a  hollow  space  surrounding  the 
cylinder,  connected  to  the  main  steam  supply  for  the  engine,  and 
filled  with  steam  at  high  pressure.  The  steam  jacket  is  supposed 
to  benefit  by  keeping  the  cylinder  walls  at  a  uniformly  high 
temperature  and  preventing  the  rapid  changes  of  temperature 
in  the  walls.  It  is  found  in  practice,  however,  that  but  little 
benefit  is  obtained  from  the  steam  jacket  because  the  changes  of 
temperature  take  place  in  only  a  thin  layer  of  metal  on  the  inner 
surfaces  of  the  cylinder  and  these  changes  of  temperature  occur 
so  rapidly  that  the  heat  from  the  jacket  does  not  have  time  to 
flow  through  the  walls  rapidly  enough  to  prevent  them.  Cylinder 
condensation  is  reduced  somewhat,  however,  by  the  presence  of 
the  jacket  because  it  maintains  a  higher  average  temperature  of 
the  cylinder  walls.  On  the  other  hand,  it  must  be  remembered 
that  whatever  heat  is  supplied  by  this  means  comes  from  the 
condensation  of  steam  in  the  jacket  and  also  that  the  presence  of 
the  jacket  makes  the  outside  diameter  of  the  cylinder  greater  and 
increases  its  surface.  The  greater  surface  of  the  cylinder  together 
with  its  higher  temperature  increases  the  amount  of  heat  lost  by 
radiation  from  the  cylinder.  Since  the  advantages  of  a  steam 
jacket  are  doubtful  and  its  presence  increases  the  cost  of  the 
engine,  it  is  not  used  as  much  now  as  formerly.  Instead,  the 
cylinders  of  the  better  classes  of  engines  are  now  simply  lagged 
with  a  nonconducting  substance  to  reduce  the  radiation  of  heat. 

The  Uniflow  Engine. — Within  recent  years  a  single  expansion 
engine  has  been  designed  with  a  view  to  reducing  the  losses  from 
cylinder  condensation.  Phis  type  of  engine  is  called  the  Uni- 
flow Engine.  A  section  of  the  cylinder  of  the  uniflow  engine  is 
shown  in  Fig.  71.  The  cylinder  contains  no  exhaust  valves  but 
a  ring  of  exhaust  ports  are  cut  in  the  middle  of  the  cylinder 
which  is  uncovered  by  the  piston  at  the  end  of  its  stroke  so  that, 
in  effect,  the  piston  is  the  exhaust  valve.  For  this  purpose,  both 
the  cylinder  and  the  piston  are  made  longer  than  in  the  ordinary 
steam  engine.  The  two  admission  valves,  A,  which  are  of  the 
Corliss  type,  are  located  in  the  cylinder  heads  and  the  steam 
spaces  over  the  valves  become  steam  jackets  for  the  heads.  The 
clearance  pocket  B  is  also  kept  filled  with  steam  so  that  the  head 
is  completely  steam  jacketed. 

After  cut-off,  the  steam  expands  behind  the  piston  as  in  the 
ordinary  types  of  engines,  but  at  the  end  of  expansion  the  piston 
uncovers  for  an  instant  the  exhaust  ports  and  the  remaining 


ACTION  OF  STEAM  IN  THE  CYLINDER         113 

pressure  in  the  cylinder  falls.  On  the  return  stroke  the  steam 
at  exhaust  temperature  and  pressure  is  caught  between  the  piston 
and  cylinder  head,  and,  as  the  piston  moves  back,  this  steam  is 
compressed  so  that  its  temperature  is  gradually  increased  and 
at  the  end  of  the  stroke  the  clearance  space  is  filled  with  steam 
at  admission  pressure.  The  temperature  of  the  steam  in  the 
clearance  space  is  increased  not  only  by  compression  but  also  by 
absorbing  heat  from  the  head  jackets.  The  result  is  that  the 
temperature  of  the  steam  in  the  clearance  space  may  be  raised 
even  higher  than  that  of  the  admission,  therefore,  when  the  ad- 
mission valve  is  opened  the  incoming  steam  meets  no  cold  sur- 
faces and  initial  condensation  is  reduced.  An  indicator  diagram 
from  a  uniflow  engine  is  shown  in  Fig.  72. 


FIG.  71. 

By  referring  to  Fig.  71  it  will  be  seen  that  each  end  of  the 
cylinder  is  provided  with  a  large  relief  valve  D  opening  into  a 
pocket  B  in  the  cylinder  head.  This  valve  serves  two  purposes: 
First,  it  is  a  relief  valve  of  large  size  which  will  relieve  the  engine 
of  any  entrained  water ;  second,  if,  when  exhausting  into  a  vacuum, 
the  vacuum  should  be  broken,  it  is  necessary  to  provide  the 
engine  with  a  larger  clearance  volume  in  order  to  prevent  exces- 
sive compression.  These  valves  open  automatically  in  case  the 
vacuum  is  broken,  and,  if  it  is  then  desired  to  run  the  engine 
noncondensing,  means  are  provided  to  back  these  valves  off  their 
seats,  thus  increasing  the  clearance  space  by  the  volume  of  the 
clearance  pockets.  The  enlargements  C,  C,  in  the  steam  passages 
are  to  provide  for  expansion  of  the  metal  without  distorting  the 
cylinder. 


114 


STEAM  ENGINES 


That  these  engines  at  least  partly  accomplish  their  purpose  is 
shown  by  the  fact  that  they  have  developed  a  horsepower  with  a 
steam  consumption  of  only  13.2  pounds  per  hour,  which  is  a 
very  good  performance  for  a  single  expansion  engine  even  when 
exhausting  into  a  vacuum  of  26  inches. 

Measuring  Cylinder  Condensation. — The  quantity  of  water 
present  in  the  cylinder  at  any  time  between  cut-off  and  release 
may  be  found  from  the  indicator  diagram  and  a  knowledge  of  the 
weight  of  steam  used  by  the  engine.  The  method  of  doing  this 
is  best  shown  by  means  of  an  actual  example.  Fig.  73  shows  an 
indicator  diagram  from  the  head  end  of  a  12"  X  24"  engine, 
with  a  clearance  on  the  head  end  of  7.9  per  cent.  The  engine 
was  running  100  r.p.m.  when  the  diagram  was  taken,  and,  by 
condensing  and  weighing  the  exhaust  for  an  hour,  it  was  found 


FIG.  72. 

• 

that  the  engine  was  using  2384.4  pounds  of  steam  per  hour.  The 
barometer  read  28.52  inches,  equivalent  to  an  atmospheric 
pressure  of  14  Ibs.  per  sq.  in.  The  spring  in  the  indicator  was 
No.  100.  By  drawing  on  the  diagram  the  "dry  steam  line" 
SS,  as  expalined  below,  the  percentage  of  water  in  the  cylinder, 
or  "quality"  of  the  steam  at  anytime  between  cut-off  and  release, 
may  be  measured  directly  from  the  diagram.  Thus  in  Fig.  73, 
the  quality  of  the  steam  at  cut-off  is  found  by  taking  the  pro- 
portion between  the  lengths  of  the  lines  AB  and  AC.  From  the 
diagram,  the  length  of  the  line  AB  is  1.0  inch  and  the  length  of 
AC  is  1.43  inches,  therefore  the  quality  of  the  steam  at  cut-off  is 

=  T-~  3  =  .70  =  70  per  cent. 

1.4o 


or,  of  the  mixture  in  the  cylinder  at  cut-off,  70  per  cent,  is  dry 
steam  and  30  per  cent,  is  water.     In  other  words,  the  initial  con- 


ACTION  OF  STEAM  IN  THE  CYLINDER        115 

densation  has  been  30  per  cent,  of  the  steam  supplied  to  the 
cylinder.  In  a  similar  manner  the  quality  of  the  steam  at  any 
other  point  F  in  the  expansion  line  is  found  by  measuring  the 
line  DE,  which  is  2v28  inches  and  the  line  DF,  which  is  2.86 
inches  and  taking  the  ratio 

DE       2.28 

Z>F  =  2^86  =  °r          per  cent* 


showing  that  the  steam  in  the  cylinder  is  dryer  at  the  point  E 
than  it  is  at  the  point  B,  a  result  that  is  to  be  expected. 

In  order  to  draw  the  dry  steam  line  it  is  first  necessary  to 
locate  the  line  of  no  pressure,  OG,  and  the  line  of  no  volume,  OH. 


M 


FIG.  73. 


The  line  OG  is  used  as  a  base  line  from  which  to  measure  absolute 
pressures  and  is  drawn  parallel  to  the  atmospheric  line,  JJ,  and  at 
a  distance  below  it  equal  to  the  atmospheric  pressure,  14  lb. 
per  sq.  in.  to  the  same  scale  to  which  the  diagram  is  drawn. 
Volume  of  steam  in  the  cylinder  is  also  measured  on  this  line. 
The  line,  OH,  is  used  for  determining  the  amount  of  steam  in  the 
clearance  volume  during  compression  and  also  to  measure  pres- 
sures. The  line  OH  is  drawn  perpendicular  to  the  atmospheric 
line  and  at  a  distance  from  the  end  of  the  diagram  equal  to  the 
volume  of  the  clearance  space,  to  the  same  scale  to  which  the 
diagram  is  drawn.  By  drawing  limiting  lines  at  the  ends  of  the 
diagram,  extending  to  the  atmospheric  line,  the  length  of  the 
diagram  is  measured  on  the  atmospheric  line  and  is  found  to  be 


116  STEAM  ENGINES 

3.34  in.  The  piston  displacement  is  .7854  X  I2  X  2  =  1.5708 
cu.  ft.  Therefore  one  inch  length  on  the  diagram  represents  a 

volume  of     '  0 .     =  .47  cu.  ft.     The  volume  of  the  clearance 
3.34 

is  .079  X  1.5708  =  .124  cu.  ft.     Therefore  the  line  OH  is  laid 

124 
off  from  the  end  of  the  diagram  a  distance  of  '—^-  =  .264  inch. 

The  weight  of  steam  in  the  clearance  space  during  compression 
is  found  by  taking  any  point  such  as  K  on  the  compression  curve 
after  the  exhaust  valve  is  closed  and  measuring  the  pressure 
and  volume  represented  by  this  point.  The  absolute  pressure  of 
the  point  K  is  found  by  measurement  to  be  40  Ib.  per  sq.  in. 
The  volume  of  one  pound  of  dry  steam  at  this  pressure  is,  from  the 
steam  table,  10.3  cu.  ft.  The  volume  of  steam  in  the  clearance 
space  is  represented  by  the  line  LK  which  is  .34  inch.  It,  there- 
fore, represents  a  volume  of  .34  X  .47  =  .1598  cu.  ft.,  and  its 

weight  is  '      »    =  .0152  pound.  /The  weight  of  steam  fed  to  each 

,    2384.4  1192.2 

end  of  the  cylinder  per  hour  is  — „ —  =  1192.2  Ibs.  or  — ™ —  = 

£  ou 

19.87  Ibs.  per  minute.  Since  the  engine  was  running  100  r.p.m., 
the  weight  of  steam  fed  to  the  engine  while  the  diagram  was  being 

19  87 
drawn  was    1 '       =  .1987  pound.     The  weight  of  steam  expand- 

1UU 

ing  in  the  cylinder  each  time  was,  therefore,  .0152  -f  .1987  = 
.2139  pound  and  it  is  for  this  weight  of  steam  that  the  dry  steam 
line  SS  must  be  drawn. 

The  dry  steam  line  SS  is  drawn  by  taking  from  the  steam  table 
the  volumes  of  one  pound  of  dry  steam  at  various  pressures  and 
multiplying  them  by  the  weight  of  steam  expanding  in  the 
cylinder  .2139  Ib.  Thus  at  an  absolute  pressure  of  150  Ib.  per 
sq.  n.  the  volume  of  one  pound  of  dry  steam  is,  from  the  steam 
table,  2.978  cu.  ft.,  therefore,  the  volume  of  .2139  Ibs.  is  .2139  X 
2.978  =  .6443  cu.  ft.  Measuring  from  0  a  distance  equal  to 

.6443 
-j=-  =    1.37   inches  the  point  Q  is  located,  which  represents 

a  volume  of  .6443  cu.  ft.  Drawing  the  line  PQ  perpendicular 
to  OG  and  of  a  length  equal  to  150  Ibs.  per  sq.  in.,  the  point 
P  is  located,  which  is  one  point  on  the  dry  steam  line.  At  an 
absolute  pressure  of  120  Ib.  per  sq.  in.,  the  volume  of  one  pound 
of  dry  steam  is,  from  the  steam  table,  3.726  cu.  ft.,  therefore,  the 
volume  of  .2139  pounds  is  .2139  X3.726  =  .7970  cu.  ft.  Meas- 


ACTION  OF  STEAM  IN  THE  CYLINDER         117 

uring  from  0  a  distance  equal  to      .,-     =  1.70  inches,  the  point 

S  is  located,  which  represents  a  volume  of  .7970  cu.  ft.  Drawing 
the  line  RS  perpendicular  to*$G  and  of  length  equal  to  120  Ib. 
per  sq.  in.  the  point  R  is  located  which  is  another  point  on  the 
dry  steam  line.  In  a  similar  manner  any  number  of  points  on 
the  dry  steam  line  may  be  found,  and  a  smooth  curve  drawn 
through  these  will  give  the  dry  steam  line  SS.  The  quality 
of  steam  in  the  cylinder  may  then  be  measured  from  this  line, 
as  explained  before.  In  order  for  this  method  of  finding  the 
quality  of  steam  in  the  cylinder  to  give  accurate  results,  there 
must  be  no  leakage  in  the  cylinder,  as  this  would  change  the  shape 
of  the  expansion  line. 


CHAPTER  IX 
STEAM  ENGINE  TESTING 

Principles. — The  usual  steam  engine  test  is  made  to  determine 
the  weight  of  steam  or  the  number  of  heat  units  which  the  engine 
consumes  per  hour  for  each  horsepower  developed  or  else  to 
determine  the  efficiency  of  the  engine.  If  possible,  the  steam 
consumption  and  efficiency  should  be  based  on  the  brake  horse- 
power of  the  engine  because  this  is  the  useful  power  of  the  engine. 
However  it  is  sometimes  impossible  or  impractical  to  obtain  the 
brake  horsepower  and,  in  this  case  the  steam  consumption  and 
efficiency  are  based  on  the  indicated  horsepower. 

As  practically  all  engines  operate  under  variable  loads,  it  is 
advisable  in  testing  them,  to  test  at  different  per  cents  of  the  full 
load  in  order  to  determine  what  performance  may  be  expected 
under  different  conditions.  For  this  purpose  it  is  convenient 
to  test  an  engine  under  one  quarter  load,  one  half  load,  three 
quarters  load,  full  load,  and  one  and  one  quarter  of  its  full  load 
capacity.  With  this  data  at  hand  a  curve  may  be  plotted  with 
brake  horsepower  or  indicated  horsepower  on  one  axis  and  steam 
consumption  or  efficiency  on  the  other  axis,  and  from  the  curve 
so  obtained,  one  may  determine  what  performance  to  expect 
from  the  engine  under  the  load  at  which  it  operates  most  of  the 
time. 

If  the  engine  is  belted  and  not  very  large,  its  brake  horse- 
power may  be  measured  with  any  of  the  forms  of  friction 
brakes  described  in  a  previous  chapter,  although  these  require 
the  use  of  a  special  pulley  designed  to  hold  water  for  cooling. 
If  the  engine  is  connected  directly  to  an  electric  generator,  it 
will  be  necessary  to  determine  separately  the  amount  of  power 
required  to  run  the  generator  without  load,  or  the  friction  load 
of  the  generator,  so  that  this  may  be  deducted  from  the  out- 
put of  the  generator  in  calculating  the  brake  horsepower  of 
the  engine.  If  the  brake  horsepower  cannot  be  determined  by 

118 


STEAM  ENGINE  TESTING  110 

one  of  these  methods,  it  will  be  necessary  to  base  the  calcula- 
tions for  steam  consumption  and  efficiency  upon  the  indicated 
horsepower.  f^ 

Steam  Consumption.— The  steam  consumption  is  best  deter- 
mined by  means  of  a  surface  condenser.  In  this  case  the  exhaust 
steam  from  the  engine  is  simply  run  into  a  surface  condenser 
where  it  is  condensed  and  the  condensate  weighed.  In  order  to 
secure  accurate  results  by  this  method  the  condenser  should  be 
free  from  leaks  and  the  condensate  should  be  cooled  to  a  tempera- 
ture that  will  prevent  its  giving  off  much  vapor,  as  otherwise 
the  loss  of  condensate  by  evaporation  will  seriously  affect  the 
results  of  the  test. 

The  steam  consumption  may  also  be  determined  by  means  of 
one  of  the  commercial  forms  of  steam  meters  which  measures 
the  weight  of  steam  passing  through  it.  If  this  method  of 
determining  the  steam  consumption  is  used,  the  steam  meter 
should  first  be  carefully  calibrated  to  insure  its  giving  accurate 
results. 

If  neither  of  the  above  methods  is  available,  it  may  be  possible 
to  isolate  the  boiler  or  boilers  supplying  the  engine,  so  that  all  of 
the  steam  generated  by  the  boiler  is  used  in  the  engine.  The 
feed  water  supplied  to  the  boiler  may  then  be  measured  and  taken 
as  the  steam  consumption  of  the  engine.  Sometimes  it  is 
necessary  for  the  boiler  used  in  this  way  to  supply  steam  for  some 
auxiliaries  such  as  feed  pumps,  etc.  In  such  a  case  it  is  necessary 
to  determine  separately  the  amount  of  steam  used  by  the 
auxiliaries.  This  may  usually  be  done,  by  condensing  the 
exhaust  steam  from  them.  In  using  this  method  of  determining 
the  steam  consumption  of  an  engine  extreme  care  should  be 
taken  to  insure  that  there  are  no  leaks,  especially  at  branches 
stopped  by  valves.  This  is  done  by  closing  all  valves  in  branches 
and  the  main  stop  valve  at  the  engine  so  that  the  main 
supply  pipe  is  open  from  the  boiler  to  the  engine  valve, 
but  closed  everywhere  else.  With  a  quiet  furnace  fire  so 
that  there  is  no  active  evaporation  the  level  of  the  water  in 
the  boiler  is  noted  from  time  to  time.  If  the  water  level  falls, 
leakage  is  taking  place  and  the  leaks  should  be  located  and 
stopped  or  else  the  rate  of  leakage  allowed  for  in  the  steam 
consumption. 

Steam  Consumption  from  Diagram. — It  is  sometimes  impos- 
sible to  find  the  weight  of  steam  used  by  an  engine  by  condensing 


120  STEAM  ENGINES 

the  exhaust  and  weighing  it,  or  by  isolating  the  boiler  and 
weighing  the  feed  water.  In  such  cases  the  weight  of  steam  used 
may  be  found  from  the  indicator  diagram,  but  this  method»should 
not  be  used  except  when  the  weight  of  steam  used  cannot  be 
found  by  any  other  method,  because  it  is  subject  to  serious 
errors  on  account  of  leakage  of  steam  into  or  out  of  the  cylinder 
and  from  one  side  of  the  piston  to  the  other. 

The  method  of  finding  the  steam  consumption  from  the  indi- 
cator diagram  may  be  illustrated  by  Fig.  73.  By  this  method 
it  is  first  necessary  to  draw  the  no  volume  line,  OH,  and  the  no 
pressure  line,  OG,  as  described  before.  If  the  barometer  reading 
is  not  known,  it  is  customary  to  draw  the  line  OG  at  a  distance 
below  the  atmospheric  line  equal  to  14.7  Ib.  per  sq.  in.  to  the 
same  scale  as  the  spring  used  in  drawing  the  diagram.  A  line 
DE  is  drawn  across  the  diagram  parallel  to  the  atmospheric 
line  and  at  a  point  near  the  end  of  the  expansion  line.  The 
weight  of  steam  represented  by  the  volume  DE  is  the  weight  which 
is  expanding  in  the  cylinder.  This  weight  minus  the  weight 
of  steam  compressed  into  the  clearance  space  is  the  weight  of 
steam  fed  to  the  cylinder  at  each  stroke.  The  length  of  the  line 
DE  is  2.66  inches  and  since  a  length  of  one  inch  on  the  diagram 
represents  a  volume  of  .47  cu.  ft.  the  volume  represented  by  the 
line  DE  is  .47  X  2.26  =  1.0622  cu.  ft.  At  the  point  E  on  the 
expansion  line,  the  steam  in  the  cylinder  has  an  absolute  pressure 
of  65  Ib.  per  sq.  in.  From  the  steam  table  one  cubic  foot  of 
steam  at  65  Ibs.  absolute  pressure  weighs  .1503  pound,  hence  the 
weight  of  steam  expanding  in  the  cylinder  is  1.0622  X  ,1503  = 
.1626  Ib. 

The  weight  of  steam  in  the  clearance  space  is  found  by  selecting 
a  point  K  on  the  compression  curve  after  the  exhaust  valve  has 
closed  and  measuring  the  pressure  and  volume  of  steam  repre- 
sented by  this  point.  This  was  done  before  in  drawing  the  dry 
steam  line,  and  it  was  found  that  the  weight  of  steam  in  the 
clearance  space  was  .0152  pound.  Therefore  the  weight  of 
steam  fed  to  the  cylinder  at  each  stroke  was  .1626  —  .0152 
=  .1474  Ibs.  Since  the  engine  was  making  200  strokes  per  min- 
ute, the  weight  of  steam  used  per  hour  as  shown  by  the  diagram 
was  .1474  X  200  X  60  =  1768.8  Ibs.  This  weight  makes  no 
allowance  for  the  condensation  in  the  cylinder,  hence  it  must 
be  corrected  by  means  of  the  values  given  in  the  following 
table. 


STEAM  ENGINE  TESTING 


121 


Percentage  of 

Part  of  steam  accounted  for  by  the  indicator  diagram 

strokes  completed 
at  qut-off 

Simple  engines  *  • 

Compound  engines 
H.P.  cylinder 

Triple  expansion: 
engines   H.P. 
cylinder 

5 

0.58 

10 

0.66 

0.74 

15 

0.71 

0.76 

0.78 

20 

0.74 

0.78 

0.80 

30 

0.78 

0.82 

0.84 

40 

0.82 

0.85 

0.87 

50 

0.86                        0.88 

0.90 

By  measurement  it  is  found  that  cut-off  in  the  above  example 
occurs  at  about  19  per  cent,  of  the  stroke,  hence  the  weight  of 
steam  from  the  diagram  should  be  divided  by  .74  or 


'     =  2390  pounds  per  hour  as  the  probable  weight  of  steam 

used  by  the  engine.  While  this  result  is  close  to  the  actual  weight 
of  steam  used,  2384  Ibs.,  it  must  be  remembered  that  this  method 
of  finding  the  weight  of  steam  used  is  liable  to  serious  error, 
sometimes  amounting  to  as  much  as  50  per  cent. 

It  is  customary  to  express  the  steam  consumption  per  horse- 
power hours  in  terms  of  the  "dry  steam  equivalent,"  that  is,  in 
terms  of  the  number  of  pounds  of  dry  steam  which  would  contain 
as  many  B.t.u.  as  is  contained  by  the  steam  of  the  quality 
actually  supplied  to  the  engine.  In  order  to  make  this  calcula- 
tion, the  quality  of  steam  supplied  to  the  engine  during  the  test 
is  measured  and  the  number  of  heat  units  in  one  pound  of  this 
steam  calculated.  This  quantity  multiplied  by  the  number 
of  pounds  of  steam  supplied  per  horsepower  gives  the  total 
number  of  heat  units  supplied  to  the  engine  per  horsepower. 
The  number  of  heat  units  per  horsepower  is  then  divided  by  the 
number  of  heat  units  in  one  pound  of  dry  steam  of  the  same 
pressure  as  that  supplied  to  the  engine  and  the  quotient  will  be 
the  number  of  pounds  of  equivalent  dry  steam  supplied  per 
horsepower. 

This  method  of  stating  the  performance  of  an  engine  forms  a 
very  satisfactory  basis  for  comparing  one  engine  with  another, 
provided  the  engines  are  operating  under  similar  conditions, 
but  the  quality  of  the  engine  cannot  be  judged  by  this  method 
of  comparison  if  one  engine  uses  superheated  steam  and  the 


122 


STEAM  ENGINES 


other  one  uses  saturated  steam.  Neither  does  a  comparison  of 
efficiencies  as  calculated  in  a  previous  paragraph  form  a  satis- 
factory basis  for  comparing  the  qualities  of  the  engines  if,  one  is 
run  condensing  and  the  other  noncondensing.  The  steam  con- 
sumption of  engines  varies  widely,  depending  upon  the  kind  of 
engine  and  the  conditions  under  which  it  is  operated.  Some 
of  the  best  performances  of  engines  that  have  been  recorded  are 
given  below. 


H.P. 

Gage 
pressure 
Ibs. 

Vacuum 
inches 

R.P.M. 

Super- 
heat 
degrees 

Lbs.  of 
dry  steam 
per  I.  H.P. 
per  hour 

Westinghouse  vertical  at 
Brooklyn,  N.  Y  

5,400 

185 

27  3 

76 

11  93 

Rockwood-Wheelock  at  Natick, 
R  I 

595 

159 

25  4 

76  4 

13  0 

McIntosh&Seymour  at 
Webster  Mass 

1  076 

123 

27  10 

99  6 

20 

12  76 

Rice  &  Sargent  at  Brooklyn, 
N  Y 

627 

151 

28  6 

121 

12  10 

Rice  &  Sargent  at  Philadelphia, 
Pa 

420 

142 

25  8 

102 

297 

9  56 

658 

150  4 

26  4 

80 

16  4 

12  03 

Leavitt  pumping  engine  at 
Chestnut  Hill  Mass 

575  7 

175  7 

27  25 

50  6 

11  20 

Duration  of  Engine  Test. — The  duration  of  a  test  will  depend 
upon  the  conditions  under  which  the  test  is  conducted  and  upon 
the  methods  used  in  making  the  different  measurements.  If 
the  engine  is  tested  under  a  brake  load  which  may  be  kept  con- 
stant, the  test  is  simplified  and  the  time  of  conducting  the  test 
shortened.  During  each  test  at  a  constant  load,  the  steam  used 
is  weighed  at  uniform  intervals  of  time,  say  ten  or  fifteen  min- 
utes. When  there  are  six  or  eight  of  these  which  are  nearly 
constant  in  amount,  the  run  may  be  discontinued  provided  the 
error  of  starting  and  stopping  is  not  large.  The  error  of  starting 
and  stopping  will  depend  upon  the  method  used  in  measuring 
the  steam  consumption.  If  the  steam  consumption  is  measured 
from  a  surface  condenser,  the  error  from  starting  and  stopping 
will  be  only  the  difference  in  the  amounts  of  condensate  in  the 
condenser  at  the  beginning  and  end  of  the  test.  This  will  be  a 


STEAM  ENGINE  TESTING  123 

relatively  small  amount.  With  a  steam  meter  used  for  measuring 
the  steam  consumption  the  error  of  starting  and  stopping  the 
test  will  be  even  smaller  thgtn  with  a  surface  condenser.  But, 
when  the  steam  consumption  is  determined  by  measuring  the 
feed  water  supplied  to  a  boiler,  the  error  of  starting  and  stopping 
the  test  will  be  large  and  the  test  must  be  conducted  for  a  greater 
length  of  time.  The  error  in  this  case  may  arise  from  a  difference 
in  level  of  the  water  in  the  boiler  at  starting  and  stopping  or  from 
a  difference  in  the  density  of  the  water  due  to  a  different  rate  of 
boiling. 

The  frequency  of  taking  indicator  diagrams  from  the  engine 
will  depend  on  how  the  load  is  varying.  With  a  constant  brake 
load  indicator  diagrams  may  be  taken  at  ten  minute  intervals, 
but  with  a  varying  load  the  intervals  should  be  from  three  to 
five  minutes.  The  object  in  any  case  is  to  get  a  fair  average  of 
the  mean  effective  pressure  or  indicated  horsepower. 

Efficiency  of  Steam  Engines. — The  term  efficiency  usually 
means  the  ratio  between  the  work  obtained  from  a  machine  and 
the  energy  supplied  to  it.  The  efficiency  of  a  steam  engine  may 
therefore  be  expressed  as: 

Work  obtained  from  the  engine 

Efficiency  =  ^ —  — ,.    ,  ,  -77- —. — 

Energy  supplied  to  the  engine 

On  this  basis  the  efficiency  of  a  steam  engine  is  very  low  on  ac- 
count of  the  large  losses  of  heat  taking  place  in  the  engine  itself 
and  of  the  large  amount  of  heat  rejected  by  the  engine  in  the 
exhaust  steam. 

In  calculating  the  efficiency  of  a  steam  engine  either  the  brake 
horsepower  or  the  indicated  horsepower  may  be  used  as  the  "work 
obtained  from  the  engine,"  but  it  should  be  stated  which  of  these 
is  used.  If  the  energy  supplied  to  the  engine  is  expressed  in 
B.t.u.  per  minute,  the  work  obtained  from  the  engine  should 
also  be  expressed  in  B.t.u.  per  minute;  this  may  be  done  by 
multiplying  the  horsepower  by  42.42.  If  the  energy  supplied 
to  the  engine  is  expressed  in  B.t.u.  per  hour,  the  horsepower 
should  be  multiplied  by  2545  to  obtain  the  work  done  by  the 
engine  per  hour  in  B.t.u. 

The  " energy  supplied  to  the  engine"  should  include  all  of  the 

heat  actually  supplied  to  the  engine,  calculated  above  the  heat  of 

the  liquid  for  the  exhaust  pressure.     It  would  not  be  fair  to  the 

engine  to  charge  it  with  the  heat  of  the  liquid  below  the  exhaust 

12 


124  STEAM  ENGINES 

pressure  because  the  engine  could  not  possibly  change  this  heat 
into  work.  Neither  should  the  engine  be  charged  with  all  of 
the  heat  in  the  steam  above  32°F.,  because  the  engine  would 
have  to  exhaust  into  a  vacuum  in  which  the  absolute  pressure 
was  only  .089  Ib.  per  sq.  in.  in  order  to  make  all  of  this  heat 
available,  and  it  is  not  possible  to  produce  and  maintain  this 
low  pressure  in  a  condenser. 

The  above  expression  for  efficiency  of  a  steam  engine  now  be- 
comes : 

B.H.P.  X  42.42 
:  W(qL  +  h  -hi) 
in  which 

E  is  efficiency  of  the  engine 

B.H.P.   is   the  brake  horsepower  of  the  engine.     (Use 

I.H.P.  if  necessary.) 

W  is  the  weight  of  steam  supplied  to  the  engine  per  minute 
q  is  the  quality  of  the  steam  supplied  to  the  engine 
L  is  the  latent  heat  of  the  steam  per  pound  at  admission 

pressure 

h  is  the  heat  of  the  liquid  per  pound  at  admission  pressure 
hi  is  the  heat  of  the  liquid  per  pound  at  exhaust  pressure 

Example.  —  An  engine  receiving  steam  at  a  pressure  of  150  Ibs.  per  sq.  in. 
absolute  and  having  a  quality  of  98  per  cent,  develops  600  I.H.P.  and  uses 
12,000  Ibs.  of  steam  per  hour.  The  exhaust  pressure  is  16  Ib.  per  sq.  in. 
absolute.  What  is  the  efficiency  of  the  engine? 

Solution.  —  The  weight  of  steam  used  per  minute  is 

-*>"-. 


The  latent  heat  of  steam  at  150  Ibs.  per  sq.  in.  absolute  pressure  is  863.2 
B.t.u. 

The  heat  of  the  liquid  at  150  Ibs.  per  sq.  in.  absolute  pressure  is  320.2 
B.t.u. 

The  heat  of  the  liquid  at  16  Ib.  per  sq.  in.  absolute  pressure  is  184.4  B.t.u. 

Therefore 

I.H.P.  X  42.42 


E  = 


W(qL  +  h  -  fci) 

600  X  42.42 


200(.98  X  863.2  +  330.2  -  184.4) 

26,452  25,452 

~  200X991.8  "  198,346 
=  .1313  or  13.13  per  cent. 

The  above  formula  for  calculating  the  efficiency  of  a  steam 
engine  is  used  only  when  the  engine  is  supplied  with  saturated 


STEAM  ENGINE  TESTING  125 

steam.  If  superheated  steam  is  supplied,  the  heat  supplied  to 
the  engine  should  include  the  heat  required  to  superheat  the 
steam. 

Efficiency  of  a  Perfect  Engine.— The  efficiency  of  an  imaginary 
perfect  engine  may  be  calculated  by  the  formula 


in  which   Ep  =  the  efficiency  of  the  perfect  engine 

TI  =  the  absolute  temperature  of  the  admission  steam 
772  =  the  absolute  temperature  of  the  exhaust  steam 

"By  absolute  temperature  is  meant  the  temperature  reckoned  from 
the  absolute  zero  of  temperature  or  the  point  below  which  it 
would  be  impossible  to  cool  any  substance.  This  point  is  located 
at  460°  below  zero  on  the  Fahrenheit  scale  of  temperatures, 
hence  to  change  Fahrenheit  temperature  to  absolute  temperature 
it  is  necessary  to  add  460°  to  the  Fahrenheit  temperature. 
If  the  engine  in  the  above  example  had  been  a  perfect  engine, 
its  efficiency  would  have  been  calculated  as  follows:  The  tem- 
perature of  steam  at  150  Ib.  per  sq.  in.  absolute  pressure  is,  from 
the  steam  table,  358.5°F.  and  its  absolute  temperature  is,  there- 
fore, 358.2  +  460  =  818.5°.  The  temperature  of  steam  at  16 
Ib.  per  sq.  in.  absolute  pressure  is,  from  the  steam  table,  216.3° 
F  and  its  absolute  pressure  is,  therefore,  216.3°  +  460  =  676.3°. 
Hence,  the  efficiency  of  the  perfect  engine  would  be 

818.5  -  676.3 
EP  =  57^H =  -1734  or  17-34  Per  cent. 

olo.O 

It  will  be  observed  that  the  efficiency  of  the  perfect  engine 
depends  only  upon  the  temperature  of  the  admission  steam  and 
of  the  exhaust  steam.  It  follows,  therefore,  that  the  efficiency 
of  the  perfect  engine  can  never  be  100  per  cent,  because  the 
maximum  temperature  of  the  admission  steam  is  limited,  and 
also  it  is  impossible  to  reduce  the  temperature  of  the  exhaust 
steam  to  the  absolute  zero. 

The  efficiency  of  a  perfect  engine,  as  calculated  above,  is  often 
used  as  a  standard  by  which  to  compare  the  efficiencies  of  dif- 
ferent engines,  the  admission  and  exhaust  temperature  being 
taken  the  same  for  both  the  perfect  and  the  actual  engine.  The 
method  of  comparison  is  to  divide  the  efficiency  of  the  actual 
engine  by  the  efficiency  of  the  perfect  engine,  the  result  being 
called  the  efficiency  ratio  or 


126  STEAM  ENGINES 

E 

Efficiency  Ratio  =  ^r 
UP 

in  which  E  =  the  efficiency  of  the  actual  engine 

Ep  =  the  efficiency  of  the  perfect  engine  taken  between 

the  same  limits  of  temperature 

The  efficiency  ratio  for  the  engine  mentioned  in  the  example 
above  is 

ET  1^1^ 

Efficiency  Ratio  =  —  =      '      =  .7514  or  75.14  per  cent. 

Mip         l/.o4 

Computations. — In  order  to  show  how  the  computations  for 
an  engine  test  are  made  and  also  to  show  a  form  for  reporting 
the  results,  the  data  and  computed  results  of  an  efficiency  test 
of  an  engine  are  given  below.  Both  the  data  taken  in  making 
the  test  and  also  the  results  computed  from  this  data  are  first 
tabulated,  the  calculated  results  being  in  heavy  face  type,  and 
following  this  the  method  of  making  the  calculations  is  shown. 

The  test  given  below  is  one  of  a  series  of  tests  made  on  an  auto- 
matic high  speed  engine  in  the  steam  laboratory  of  the  University 
of  Wisconsin  to  determine  its  economy,  thermal  efficiency, 
and  mechanical  efficiency.  During  each  test  the  engine  carried  a 
practically  constant  load  made  by  a  Prony  brake  similar  to  that 
shown  in  Fig.  69.  The  steam  consumption  was  measured  by 
passing  the  exhaust  steam  from  the  engine  into  a  surface  conden- 
ser operated  at  atmospheric  pressure,  where  it  was  condensed 
and  weighed.  This  method  of  loading  the  engine  and  deter- 
mining the  steam  consumption  made  it  possible  to  secure  suffi- 
ciently accurate  results  by  means  of  a  test  of  twenty  minutes 
duration. 

Report  of  Steam  Engine  Test 
Item. 

1.  Date,  November  15,  1915. 

2.  Kind  of  Engine.     Weston  high  speed  automatic  noncon- 

densing  single  cylinder  and  simple  valve. 

3.  Dimensions  10"  X  13"  Piston  Rod  1%". 

4.  Rated  horsepower 75  I.H.P. 

5.  Horsepower  constant,  head  end 0.002678 

Horsepower  constant,  crank  end 0.00260 

6.  Atmospheric  pressure,  in  mercury 28.768 

Atmospheric  pressure,  Ibs.  per  sq.  in 14.12 

7.  Length  of  brake  arm 5  ft. 

8.  Brake  constant 0.000962 

9.  Duration  of  test..  20  min. 


STEAM  ENGINE  TESTING  127 

Item. 

10.  Average  R.P.M  ........  .,  ..........................  243 

11.  Average  steam  line  pressure,  Ibs.  gage  ..................    116 

Average  steam  line  pressufe,  Ibs.  absolute  ..............   130.12 

12.  Average  M.E.P.  from  indicator  diagrams  H.E  .........   62.83 

Average  M.E.P.  from  indicator  diagrams  C.E  ..........   61.61 

13.  Total  weight  condensed  steam,  Ibs  ....................    1113.5 

14.  Weight  of  condensed  steam,  Ibs.  per  hour  ..............   3340.5 

15.  Quality  of  steam,  per  cent  ..............  ..............  98.6 

16.  Dry  steam  supplied  per  hour,  Ibs  .....................   3303.7 

17.  Brake  load,  net,  Ibs  .................................   306 

18.  Brake  horsepower  ...................................   70.78 

19.  Indicated  horsepower,  head  end  ......................   39.36 

20.  Indicated  horsepower  crank  end  ......................  37.37 

21.  Indicated  horsepower,  total  ..........................   76.73 

22.  Friction  horsepower  .................................   6.96 

23.  Mechanical  efficiency,  per  cent  .......................  92.2 

24.  Dry  steam  per  I.H.P.  hr.,  Ibs  .....................  ....  43.06 

25.  Dry  steam  per  B.H.P.  hr.,  Ibs  ......  •  ...................  46.8 

26.  Exhaust  pressure,  Ibs.  absolute  from  indicator  diagrams  .    18 

27.  Thermal  efficiency,  on  I.H.P.  per  cent  ..........  ,  ......   6.916 

28.  B.t.u.  supplied  per  I.H.P.  hr.  above  exhaust  pressure.  .  .  .  43,016 

Calculating  Results.  —  Item  5.  —  The  formula  for  calculating 
the  indicated  horsepower  is 

THP       -    Plan 
33,000 

which  gives  the  indicated  horsepower  developed  on  one  side  of 
the  piston  if  n  in  the  formula  is  the  revolutions  per  minute.  For 
any  particular  engine  the  length  of  stroke  I  is  a  constant  quantity; 
the  area  of  the  piston,  a,  is  constant;  and  the  quantity  33,000 

is  constant.     Therefore  00  ^^  will  be  a  constant  quantity  and 

66,000 

it  is  this  part  of  the  horsepower  formula  that  is  called  the  "horse- 
power constant"  or 

H.P.  constant  = 


The  reason  for  calculating  the  horsepower  constant  separately 
instead  of  calculating  the  horsepower  directly  is  that  it  saves 
considerable  time  when  the  horsepower  must  be  calculated  a 
large  number  of  times  from  indicator  diagrams.  The  mean 
effective  pressure  from  these  diagrams  will  vary  slightly,  as  will 
also  the  number  of  revolutions  per  minute.  If  the  horsepower 
constant  is  calculated  separately,  this  part  of  the  calculations 


128  '  STEAM  ENGINES 

need  be  done  only  once  because  then  the  horsepower  may  be 
calculated  for  any  M.E.P.  and  R.P.M.  by  merely  multiplying 
the  engine  constant  by  the  M.E.P.  and  the  R.P.M.  It  is 
necessary  to  calculate  the  horsepower  constant  for  each  end  of 
the  cylinder  because  the  area  of  the  piston  is  larger  on  the  head 
end  than  on  the  crank  end,  since  the  area  of  the  piston  rod  cuts 
off  some  of  the  area  of  the  piston. 

13 

For  this  engine  the  length  of  stroke  is  13  in.  or  ^  ft.  and  the 

J.^ 

area  of  the  piston  is  102  X  .7854  =  78.54  sq.  in.  The  head  end 
horsepower  constant  is  therefore 

H.P.  constant,  head  end  =  =  =  -002578 


In  calculating  the  horsepower  constant  for  the  crank  end  it  is 
necessary  to  deduct  the  area  of  the  piston  rod  from  the  area  of 
the  piston.  The  area  of  the  piston  rod  is 

1.752  X  .7854  =  2.405  sq.  in. 
As  the  area  of  the  piston  is  78.54  sq.  in.,  the  net  effective  area^is 

78.54  -  2.405  =  76.13  sq.  in. 
Therefore  the  crank  end  horsepower  constant  is 

H.P.  constant,  crank  end  =  33-^  =  jf£  =  -°025 


Item  6.  —  The  atmospheric  pressure  is  read  on  a  barometer  and 
is  expressed  in  inches  of  mercury.  To  express  the  atmospheric 
pressure  in  pounds  per  square  inch  it  is  necessary  to  multiply  by 
.4908  or 

Atmospheric  pressure,  Ibs.  per  sq.  in.  =  28.768  X  .4908 
=  14.12. 

Item  8.  —  The  brake  constant  is  a  constant  quantity  similar  to 
the  "  horsepower  constant"  but  relating  to  the  Prony  brake  and 
it  is  used  for  lessening  the  work  of  calculating  the  brake  horse- 
power. The.  formula  used  for  calculating  the  brake  horsepower 
with  a  Prony  brake  is 

2*RWN 

33,000 
In  which 

R  =  the  length  of  the  brake  arm  in  feet 
W  =  the  net  weight  in  pounds  registered  by  the  brake 
and 

N  =  the  number  of  revolutions  per  minute. 


STEAM  ENGINE  TESTING  129 

For  any  given  engine  test  the  weight  registered  by  the  brake  and 
the  R.P.M.  may  vary  but  t^ie  other  parts  of  the  formula  will 
remain  constant.  Therefore  i 


Brake  Constant  =  -  =  .000952. 


Item  14.  —  Item  13  gives  the  weight  of  condensed  steam  in 
20  minutes,  therefore  the  weight  of  condensed  steam  in  one  hour 
will  be 

Item  13  X  2g  or 

60 
1113.5  X  ~  =  3340.5 

Item  16.  —  The  number  of  heat  units  in  one  pound  of  steam 
supplied  to  the  engine  is  found  by  the  formula 

qL  +  h 
in  which     q  =  the  quality  of  the  steam 

L  =  the  latent  heat  per  pound 
and        h  =  the  heat  of  the  liquid  per  pound. 
Substituting  in  this  formula  from  the  steam  table  the  va'iues  for 
130.12  Ib.  per  sq.  in.  absolute 

L  =  872.2  and  h  =  319.4 

or  B.t.u.  per  Ib.  =  qL  +  h  =  .985  X  872.2  +  319.4  =  1178.5 
The  total  amount  of  heat  supplied  to  the  engine  in  one  hour 
equals  the  weight  of  steam  actually  supplied  per  hour  multiplied 
by  1178.5  or 

3340.5  X  1178.5  =  3936779.25  B.t.u. 

If  the  steam  had  been  perfectly  dry,  it  would  have  contained 
L  -f-  h  heat  units  per  pound,  or 

L  +  h  =  872.2  +  319.4  =  1191.6  B.t.u.  per  pound 
Therefore  the  equivalent  weight  of  dry  steam  supplied  per  hour  is 

3936779.25  -f-  1191.6  =  3303.7  Ibs.  per  hour. 
Item  18.  —  As  explained  before  the  brake  horsepower  is  found 
by  multiplying  together  the  net  brake  load  in  pounds  (Item  17), 
the   R.P.M.    (Item    10),   and   the  brake  constant  (Item  8)  or 

B.H.P.  =  306  X  243  X  .000952  =  70.78 
Item   19.  —  In   a  similar  manner   the  indicated   horsepower, 
head  end,  is  the  product  of  the  M.E.P.,  head  end  (Item  12),  the 
R.P.M.  (Item  10)  and  the  head  end  horsepower  constant  (Item 
5)  or 


130  STEAM  ENGINES 

Head  end  I.H.P.  =  62.83  X  243  X  .002578^=  39.36 
Crank  end  I.H.P.  =  61.51  X  243  X  .0025  =  37.37 
Total  I.H.P.  =  39.36  +  37.37  =  76.73 

Item  22.  —  The  friction  horsepower  is  equal  to  the  difference 
between  the  indicated  horsepower  and  the  brake  horsepower  or 

Item  22  =  Item  21  -  Item  18 
Friction  H.P.  =  76.73  -  70.78  =  5.95 

Item  23.  —  The  mechanical  efficiency  is  equal  to  the  brake 
horsepower  divided  by  the  indicated  horsepower  or 

Mechanical  efficiency  =  JT  -  oT  =  75  73  =  *^22  or  92'2  per 
cent. 

Item  24.  —  The  dry  steam  per  I.H.P.  per  hour  is  equal  to  the 
dry  steam  per  hour  (Item  16)  divided  by  the  indicated  horse- 
power (Item  21)  or 

oono  7 

Dry  steam  per  I.H.P.  hr.  =  -=^  =  43.06 

/  D.  /o 

Item  25.  —  In  a  similar  manner  the  dry  steam  per  B.H.P.  hr. 
is  equal  to  the  dry  steam  per  hour  divided  by  the  B.H.P.  or 

3303  7 
Dry  steam  per  B.H.P.  hr.  =  JQ-^Q   =  46.8 

Item  27.  —  The  thermal  efficiency  of  the  engine,  based  on  the 
I.H.P.  may  be  calculated  by  the  formula 

__  .  I.H.P.  X  42.42 

sW(qL  +  h-hj 
in  which  I.H.P.  is  the  indicated  horsepower  (Item  21) 

W  is  the  actual  weight  of  steam  supplied  to  the  engine 

per  minute  or  Item  14  -T-  60 
q  is  the  quality  of  the  steam  supplied  to  the  engine, 

Item  15 
L  is  the  latent  heat  per  pound  of  the  steam  supplied  to 

the  engine 
h  is  the  heat  of  the  liquid  per  pound  of  the  steam  sup- 

plied to  the  engine 
hi  is  the  heat  of  the  liquid  per  pound  of  steam  at  exhaust 

pressure 
In  this  case  I.H.P.  =  76.73 


W  „  =  55.675 


STEAM  ENGINE  TESTING  131 

•  q  =  .985      . 
L  =  872.2' 
h  =  319.4  * 
hi  =  190.5 
Therefore 

Fffi  .  =  76.73  X  42.42 

55.675  (.985  X  872.2  +  319.4  -  190.5) 

3254.88 '          3254.88 
;    "  55.675  X  988       55006.9 
or  5.916  per  cent. 

Item  28. — In  calculating  Item  27  it  was  seen  that  the  number 
of  heat  units  in  one  pound  of  steam  above  exhaust  pressure  as 
actually  supplied  was  988.  Since  there  was  supplied  to  the  en- 
gine 3340.5  Ibs.  of  steam  per  hour,  the  number  of  heat  units 
supplied  per  hour  above  exhaust  pressure  was 
988  X  3340.5  =  3,300,414 

and  as  the  I.H.P.  was  76.73  the  number  of  heat  units  supplied 
per  I.H.P.  per  hour  above  exhaust  pressure  was 
3300414 


76.73 


=  43015 


Duty  of  Pumps. — The  performance  of  pumping  engines  is 
usually  stated  in  terms  of  the  number  of  foot-pounds  of  work 
performed  by  the  water  piston  of  the  pump  per  thousand  pounds 
of  dry  steam,  or  per  million  B.t.u.  consumed  by  the  engine. 
The  performance,  stated  in  this  way,  is  called  the  duty  of  the 
pumping  engine;  thus, 

^  Foot-pounds  of  work  done 

Duty  =  lltr  •  T\L    f  j  ^-j  X  1000  or 

Weight  of  dry  steam  used 

_  Foot-pounds  of  work  done 

Duty  =  ,T7  .  ^    ,  ,  3  X  1,000,000 

Weight  of  dry  steam  used 

In  using  the  above  formulas  it  is  to  be  understood  that  the  foot- 
pounds of  work  and  the  weight  of  steam  or  number  of  B.t.u. 
consumed  are  to  be  taken  for  the  same  periods  of  time. 

Example. — A  compound  pump  uses  80  pounds  of  steam  per  I.H.P.  per 
hour  and  develops  48  I.H.P.  The  pump  receives  steam  at  an  absolute 
pressure  of  135  pounds  per  sq.  in.  and  exhausts  against  an  absolute  pressure 
of  17  Ibs.  per  sq.  in.  The  quality  of  the  steam  delivered  to  the  pump  is  97 
per  cent.  The  capacity  of  the  pump  is  400  gallons  per  minute  and  pumps 
against  a  pressure  of  175  pounds  per  sq.  in.  Calculate  the  duty  of  the 
pump,  on  the  dry  steam  and  on  the  heat  unit  bases. 
13 


132  STEAM  ENGINES 

Solution.  —  175  Ib.  per  sq.  in.  pressure  is  equivalent  to 

175  X  2.3  =  402.5  feet  head 
400  gallons  is  equivalent  to 

400  X  8.33  =  3332  pounds 
Work  done  by  pump  per  minute 

=  3332  X  402.5  =  1,341,130  foot-pounds 
Work  done  by  pump  per  hour 

=  1,341,130  X  60  =  80,467,800  foot-pounds 
Heat  units  in  one  pound  of  wet  steam  above  32° 

=  (.97  X  869.9  +  321.7)  =  1165.5  B.t.u. 
Heat  units  in  one  pound  of  dry  steam  above  32° 

=  1191.6  B.t.u. 
Weight  of  wet  steam  used  per  hour 

=  80  X  48  =  3840  pounds 
Equivalent  weight  of  dry  steam  used 


3840  X  ~p     =  3755  pounds 
Duty  =      >8°Q  X  1000  =  21,423,800  foot-pounds 


Heat  units  in  one  pound  of  steam  above  heat  of  liquid  at  exhaust  pressure 

=  1165.5  -  187.5  =  978.0. 
Heat  supplied  to  pump  per  hour  above  heat  of  liquid  at  exhaust  pressure 

=  978.0  X  3840  =  3,754,520  B.t.u. 

80  467  800  • 

Duty  =  3  754  52Q    X  1,000,000  =  21,432,247  foot-pounds. 


CHAPTER  X 
THE  SLIDE  VALVE 

Steam  and  Exhaust  Lap. — The  valves  controlling  the  distribu- 
tion of  steam  to  the  cylinder  are  the  most  important  parts  of  a 
steam  engine,  because  both  the  smooth  running  of  the  engine  and 
the  economical  use  of  steam  depend  largely  upon  them. 

Of  the  different  kinds  of  valves  used  on  engines,  the  ordinary 
slide  valve  is  the  most  important.  The  slide  valve  combines  in 
one  valve  the  office  of  admission  and  exhaust  for  both  ends  of  the 
cylinder;  it  is  used  on  a  greater  variety  of  engines  than  any  other 
form  of  valve;  and  the  principles  underlying  the  operation  of  the 
slide  valve  are  also  the  principles  underlying  the  operation  of 
other  types  of  valves.  For  these  reasons  a  thorough  study  of 
the  slide  valve  will  be  made  before  considering  other  types 

The  operation  of  the  slide  valve  is  described  in  Chapter  1, 
and  the  valve  and  its  mechanism  are  illustrated  in  Figs.  2  and  4. 
The  eccentric  which  moves  the  valve  backward  and  forward 
gives  it  the  same  motion  as  would  a  crank,  and  in  fact  it  is  equiva- 
lent in  all  respects  to  a  crank  having  a  length  equal  to  the  dis- 
tance from  the  center  of  the  shaft  to  the  center  of  the  eccentric. 
This  distance  is  called  the  eccentricity. 

The  position  of  a  line  connecting  the  center  of  the  shaft  with 
the  center  of  the  eccentric  also  represents  the  position  of  the 
eccentric  or  the  position  of  the  equivalent  crank.  The  distance 
which  the  valve  travels  in  going  from  one  end  of  its  stroke  to 
the  other  is  called  the  valve  travel.  If  the  motion  of  the  eccen- 
tric is  communicated  directly  to  the  valve,  the  valve  travel  will 
equal  twice  the  eccentricity. 

When  a  valve  is  at  the  middle  pt)int  of  its  travel,  in  which 
position  the  eccentric  will  be  vertical  to  the  center  line  of  the 
engine,  the  valve  is  in  mid-position.  This  position  of  a  valve  is 
used  as  a  reference  point  from  which  the  parts  of  the  valve  and 
also  its  different  positions  are  measured.  The  cross  section 
of  a  slide  valve  in  its  mid-position  is  shown  in  Fig.  74.  When 
it  is  in  this  position,  the  length  from  the  outer  edges  of  the  ports 
14  133 


134 


STEAM  ENGINES 


P  and  P  to  the  outer  edges  A  and  A  of  the  valve  is  called  the 
outside  lap.  The  outside  lap  is  shown  by  the  dimensions  0  and 
0.  It  is  not  necessary  that  the  outside  lap  at  one  end  of  the  valve 
be  equal  to  that  at  the  other  end,  and,  in  fact,  they  are  usually 
unequal,  as  will  be  explained  later.  The  distance  from  the  inside 
edges  of  the  ports  P  and  P  to  the  inner  edges  B  and  B  of  the  valve 
is  called  the  inside  lap.  The  inside  lap  is  shown  by  the  dimen- 
sions /  and  /.  The  inside  laps  of  a  valve  are  usually  unequal. 
The  width  of  the  ports,  or  the  distance  /,  is  usually  the  same  for 
both  ends  of  the  cylinder. 

When  steam  is  admitted  past  the  outer  edge  of  the  valve,  the 
outside  lap  is  usually  called  the  steam  lap  and  the  inside  lap 
the  exhaust  lap.  When  steam  is  admitted  from  inside  the  valve 


FIG.  74. 

and  exhausted  past  the  outer  edge,  as  is  sometimes  done,  the 
inside  lap  is  called  the  steam  lap  and  the  outside  lap  is  called 
the  exhaust  lap.  The  steam  lap  is  usually  much  greater  than  the 
exhaust  lap,  hence  a  valve  designed  for  outside  admission  will 
not  distribute  the  steam  properly,  if  it  is  used  for  inside  admission. 
Unless  otherwise  mentioned  the  outside  lap  will  be  considered 
as  the  steam  lap  as  this  is  the  more  usual  arrangement.  • 

Valve  Without  Laps. — A  form  of-  slide  valve  often  used  on 
small  direct  acting  steam  pumps  is  shown  in  Fig.  75.  It  will  be 
observed  that  this  valve  differs  from  the  one  shown  in  Fig.  74 
in  having  neither  steam  nor  exhaust  laps,  the  width  of  the  valve 
being  just  equal  to  the  width  of  the  ports.  In  the  position  shown 
in  Fig.  75  the  valve  is  in  its  mid-position  and,  since  it  has  neither 
steam  nor  exhaust  lap,  it  will  open  the  port  to  admission  on  one 


THE  SLIDE  VALVE  135 

end  and  to  exhaust  on  the  other  if  the  valve  moves  ever  so  little 
to  either  side  of  its  mid-position.  If  this  kind  of  slide  valve  were 
to  be  used  on  a  steam  engine^it  would  have  to  be  set  so  as  to  be 
in  its  mid-position  when  the  piston  was  at  the  end  of  its  stroke. 
This  would  require  that  the  eccentric  be  placed  90°  from  the 
crank  as  shown  at  the  right  of  Fig.  75,  where  Oc  represents  the 
position  of  the  crank  and  Oe  the  position  of  the  eccentric.  With 
the  valve  in  its  mid-position  and  the  piston  at  the  end  of  its 
stroke,  steam  will  begin  to  be  admitted  to  the  cylinder  as  soon  as 
the  piston  starts  forward  and  the  valve  will  remain  open  until 
the  piston  has  reached  the  end  of  its  stroke,  thus  giving  admission 
throughout  the  entire  stroke.  At  the  end  of  the  stroke  the  valve 
will  close  the  admission  and  open  the  exhaust,  and  exhaust  will 
occur  throughout  the  entire  return  stroke  of  the  piston.  It 


FIG.  75. 

should  be  noted  that  the  eccentric  may  be  set  either  in  the  posi- 
tion Oe,  Fig.  75,  or  in  the  position  Od,  and  the  engine  will  run 
either  in  a  clockwise  or  counterclockwise  direction  of  rotation, 
depending  upon  the  direction  in  which  it  starts.  The  only 
requirement  in  setting  the  eccentric  for  a  valve  with  no  laps 
is  that  it  must  be  placed  90°  from  the  crank. 

An  indicator  diagram  taken  from  an  engine  fitted  with  a  valve 
having  neither  steam  nor  exhaust  lap  will  be  simply  a  rectangle,  as 
shown  in  Fig.  76.  This  diagram  shows  that  steam  is  admitted 
to  the  cylinder  during  the  entire  stroke,  being  released  at  the 
end  of  the  stroke  without  having  expanded.  Exhaust  also  occurs 
during  the  entire  return  stroke  and  none  of  the  steam  is  com- 
pressed near  the  end  of  the  stroke. 

A  valve  without  laps  is  suitable  for  small  direct  acting  pumps 
because  the  load  on  these  pumps  is  a  constant  water  pressure  and 
the  steam  pressure  on  the  piston  must  therefore  be  constant 
throughout  the  entire  stroke  in  order  to  overcome  the  constant 


136 


STEAM  ENGINES 


water  pressure.  Moreover,  the  constant  load  on  the  piston 
serves  to  bring  it  to  rest  at  the  end  of  the  stroke  without  shock 
at  the  low  speeds  at  which  such  pumps  are  usually  run,  hence 
no  compression  is  necessary.  A  valve  of  this  kind  would  not, 
however,  be  suitable  for  a  steam  engine  because  it  would  be 
uneconomical  in  the  use  of  steam  since  the  expansive  force  of  the 
steam  would  not  be  used,  and  also  because  the  high  speed  of  the 
engine  makes  compression  necessary  if  the  engine  is  to  run 
smoothly.  It  is  not  necessary  to  admit  steam  to  the  cylinder 
of  an  engine  throughout  the  entire  stroke  because  the  engine 
has  a  flywheel  which  stores  up  energy  in  the  first  part  of  the 
stroke,  giving  it  out  again  in  the  last  part  of  the  stroke  when  the 


ADMISSION 


EXHAUST 


FIG.  76. 

steam  pressure  is  small  and,  by  this  means,  causing  the  engine 
shaft  to  rotate  at  a  uniform  speed. 

Valves  With  Lap. — A  valve  that  has  steam  lap  will  keep  the 
port  closed  against  the  admission  of  steam  until  the  valve  has 
moved  from  its  mid-position  a  distance  equal  to  the  steam  lap. 
The  valve  shown  in  Fig.  77  is  an  outside  admission  valve  and  it 
has  moved  to  the  right  of  its  mid-position  a  distance  equal  to  the 
steam  lap.  In  this  position  the  valve  is  just  on  the  point  of 
admitting  steam  to  the  head  end  of  the  cylinder.  If  the  valve 
now  moves  to  the  right  steam  will  be  admitted  to  the  cylinder 
and  admission  will  continue  until  the  valve  moves  to  the  left 
and  returns  to  the  position  shown  in  Fig.  77  when  the  port  will 
be  closed.  The  port  will  then  remain  closed  and  the  steam  will 
expand  until  the  valve  moves  far  enough  to  the  left  to  bring 
the  inner  edge  of  the  valve  in  line  with  the  inner  edge  of  the  port. 


THE  SLIDE  VALVE 


137 


A  further  movement  of  the  valve  to  the  left  uncovers  the  port 
for  exhaust,  which  continues  until  the  valve,  in  moving  to  the 
right  on  its  forward  stroke,  reaches  a  position  where  the  inner 
edge  of  the  valve  is- again  inline  with  the  inner  edge  of  the  port. 
As  the  valve  continues  its  movement  to  the  right  the  port  remains 
closed  and  the  steam  in  the  cylinder  is  compressed.  Compres- 
sion continues  until  the  valve,  still  moving  towards  the  right, 
reaches  a  position  in  which  the  outer  edge  of  the  valve  is  in  line 
with  the  outer  edge  of  the  port,  when  admission  again  occurs. 
It  is  thus  seen  that  the  purpose  in  having  steam  and  exhaust  laps 
is  to  permit  the  steam  to  be  expanded  and  compressed. 

Position  of  Crank  and  Eccentric. — In  Fig.  77  the  position  of  the 
valve  is  such  that  steam  is  about  to  be  admitted  to  the  head  end 


FIG.  77. 

of  the  cylinder  and  the  piston  is  shown  at  the  beginning  of  its 
forward  stroke.  If  the  valve  is  connected  to  the  eccentric  with- 
out the  use  of  a  rocker  arm,  the  corresponding  positions  of  the 
crank  and  eccentric  will  be  as  shown  in  the  diagram  at  the  right 
of  Fig.  77,  in  which  0  represents  the  center  of  the  shaft,  OC  the 
position  of  the  crank,  and  OE  the  position  of  the  eccentric. 
When  the  valve  is  in  its  mid-position  the  eccentric  is  vertical, 
in  the  position  of  the  line  OA,  but  in  the  position  shown  at  OE 
it  has  been  moved  around  on  the  shaft  in  a  clockwise  direction 
enough  to  move  the  valve  through  a  distance  equal  to  its  steam 
lap,  which  is  equal  to  the  distance  OL  on  the  diagram.  The 
distance  OL  on  the  diagram  represents  the  displacement  of  the 
valve  from  its  mid-position. 

With  the  positions  of  crank  and  eccentric  as  shown  in  Fig.  77 
if  the  shaft  turns  in  a  clockwise  direction,  as  shown  by  the  arrow, 
the  piston  will  move  forward  and  the  valve  will  move  to  the 


138 


STEAM  ENGINES 


right,  admitting  the  steam  behind  the  piston.  This  will  push 
the  piston  forward  and  cause  the  engine  to  run.  If,  however, 
an  attempt  is  made  to  start  the  engine  by  turning  the  shaft  in  a 
direction  opposite  to  that  indicated  by  the  arrow,  or  in  a  counter- 
clockwise direction,  the  valve  will  be  moved  to  the  left  and  prevent 
the  admission  of  steam.  It  is  seen  then  that,  with  the  positions 
of  crank  and  eccentric  as  shown,  the  engine  can  run  only  in  a 
clockwise  direction.  //  there  is  no  rocker  arm,  the  direction  of  ro- 
tation of  the  engine  will  be  such  that  the  crank  follows  the  eccentric. 
If  it  was  desired  to  have  the  engine  under  consideration  run  in  a 
counterclockwise  direction  the  eccentric  would  have  to  be  set 
in  the  position  OF,  the  crank  being  at  OC. 

VALVE.    ROD 


PIVOT 


ECCENTRIC   ROD 


FIG.  78. 

Sometimes  the  shape  of  the  engine  is  such  that  the  valve  and 
eccentric  are  not  in  line  with  each  other  and  the  motion  is  trans- 
mitted from  the  eccentric  to  the  valve  through  a  rocker  arm  as 
shown  in  Fig.  78.  If  the  valve  rod  and  eccentric  rod  are  both 
on  the  same  side  of  the  pivot  the  motion  of  the  valve  will  be  in 
the  same  direction  as  if  the  valve  rod  was  connected  directly 
to  the  eccentric  rod,  but  if  they  are  connected  on  opposite  sides 
of  the  pivot,  as  shown  in  Fig.  78,  the  valve  will  move  in  the  oppo- 
site direction  to  that  in  which  it  would  move  if  directly  connected. 
Since  a  rocker  arm  pivoted  between  valve  and  eccentric  rods 
reverses  the  motion  of  the  valve,  the  position  of  the  eccentric 
with  respect  to  that  of  the  crank  will  be  as  indicated  by  the  line 
OF  of  the  diagram  at  the  right  of  Fig.  77  for  a  clockwise  direction 
of  rotation.  In  other  words,  the  use  of  such  a  rocker  arm  requires 
that  the  eccentric  be  set  to  follow  the  crank. 

Lead. — In  Fig.  77  the  valve  is  set  to  admit  steam  to  the  cylinder 


THE  SLIDE  VALVE  139 

just  at  the  beginning  of  the  stroke.  If  a  valve  is  set  in  this  way 
there  will  be  considerable  drop  in  steam  pressure  during  admis- 
sion, or  "wire  drawing.'*  The  effect  of  this  on  the  indicator 
diagram  is  shown  in  Fig.  7{f  in  which  the  reduction  in  the  admis- 
sion pressure  is  indicated  by  the  drop  in  the  admission  line. 
Since  this  reduces  the  area  of  the  diagram  it  shows  that  the  power 
of  the  engine  is  reduced.  It  also  reduces  the  efficiency  of  the 
engine  because  the  range  of  pressure  through  which  the  steam 
may  be  expanded  is  reduced. 

An  inspection  of  the  diagram  at  the  right  of  Fig.  77  will  show 
that,  if  the  shaft  rotates  with  a  uniform  speed,  the  valve  travels 
fastest  when  it  reaches  mid-position  and  the  piston  travels  fastest 


._____BpU.ER  _f  REASSURE 


FIG.  79. 

when  it  reaches  mid-stroke,  the  speed  of  each  increasing  during 
the  first  part  of  its  stroke  and  decreasing  during  the  last  part. 
This  diagram  also  shows  that,  at  the  beginning  of  the  piston 
stroke,  the  speed  of  the  valve  is  decreasing  and  the  speed  of  the 
piston  is  increasing.  The  result  is,  that  if  the  valve  is  set  to 
open  just  at  the  beginning  of  the  piston  stroke,  steam  cannot 
flow  into  the  cylinder  through  the  narrow  opening  fast  enough 
to  maintain  full  pressure  behind  the  piston,  hence  the  pressure 
in  the  cylinder  drops. 

In  order  to  prevent  excessive  drop  in  pressure  during  admission 
the  valve  is  set  so  as  to  open  slightly  before  the  end  of  the  exhaust 
stroke  thus  insuring  enough  port  opening  to  allow  free  admission 
when  the  piston  starts  forward.  The  amount  which  the  port  is 
open  for  admission  when  the  piston  is  at  the  end  of  its  stroke  is 
called  the  lead  of  the  valve.  The  amount  of  lead  which  a  valve 
should  have  depends  upon  the  size  of  the  cylinder  and  speed  of 
the  engine,  being  larger  for  high  speeds  and  large  cylinders  and 
smaller  for  slow  speeds  and  small  cylinders. 


140 


STEAM  ENGINES 


The  cushioning  of  the  piston  depends  to  a  certain  extent  upon 
the  lead  as  a  large  lead  gives  an  early  opening  of  the  valve  with 
an  early  admission  of  high  pressure  steam  against  which  the 
piston  must  advance  at  the  end  of  the  exhaust  stroke. 

A  valve  set  with  lead  is  shown  in  Fig.  80,  the  relative  positions 
of  the  crank  and  eccentric  being  shown  in  the  diagram  to  the 
right  of  the  figure.  In  this  illustration  the  piston  is  at  the  head 
end  of  its  stroke  and  the  valve  is  open  an  amount  I  for  the  admis- 
sion of  steam  to  the  head  end.  The  distance  I,  in  this  case  is  the 
lead.  It  will  be  observed  that,  in  the  position  shown,  the  valve 
has  moved  to  the  right  of  its  mid-position  a  distance  equal  to 
the  steam  lap  plus  the  lead. 

In  the  diagram  at  the  right  of  Fig.  80  the  crank  OC  is  shown  on 
dead  center  to  correspond  with  the  position  of  the  piston.  The 


FIG.  80. 

line  OE  shows  the  position  of  the  eccentric  and  the  distance  OL 
shows  the  displacement  of  the  valve  from  its  mid-position.  If 
the  valve  was  set  without  lead  the  position  of  the  eccentric  would 
be  OE' ,  the  distance  OL'  being  equal  to  the  steam  lap.  Since 
the  distance  OL  is  equal  to  the  steam  lap  plus  the  lead,  the 
distance  L'L  is  equal  to  the  lead  of  the  valve.  It  will  be  observed 
that  when  the  valve  has  lead,  the  eccentric  must  be  moved 
around  on  the  shaft  far  enough  to  displace  the  valve  from  its 
mid-position  a  distance  equal  to  the  steam  lap  plus  the  lead  when 
the  crank  is  on  center. 

Angle  of  Advance. — For  an  outside  admission  valve  connected 
directly  to  the  eccentric,  the  eccentric  must  be  set  so  that  it 
leads  the  crank.  When  the  crank  is  on  center  the  valve  must 
also  be  displaced  to  the  right  of  its  mid-position  a  distance  equal 
to  the  steam  lap  plus  the  lead.  These  two  conditions  determine 
the  position  of  the  eccentric  with  respect  to  that  of  the  crank. 


THE  SLIDE  VALVE 


141 


If  the  eccentric  was  in  the  position  OA,  Fig.  80,  when  the  crank 
was  on  center,  the  valve  would  be  in  its  mid-position,  hence  the 
eccentric  must  be  moved"  forward  through  the  angle  AOE  in 
order  to  displace  the  valve  from  its  mid-position  a  distance  equal 
to  the  steam  lap  plus  the  lead.  This  makes  the  angle  between 
the  crank  and  eccentric,  which  is  called  the  crank  angle,  greater 
than  90°.  The  angle  AOE  is  called  the  angle  of  advance  and  it 
is  the  angle,  in  excess  of  90°,  between  the  crank  and  eccentric. 
The  angle  of  advance  is  usually  about  20°  to  30°  but  its  amount 
will  depend  upon  the  steam  lap  and  the  lead  which  it  is  desirable 
to  give  the  valve.  The  angle  of  advance  is  important  in  valve 
setting,  as  will  be  shown  later,  because  it  is  the  only  thing  about 
the  valve  mechanism  besides  the  length  of  the  valve  rod  which 
may  be  adjusted. 


— ir* 


FIG.  81. 


Inside  Admission  Valve. — Many  valves,  especially  of  the  piston 
type,  are  designed  to  admit  steam  from  the  inside  and  exhaust 
past  the  outer  edge  of  the  valve.  In  this  case  the  inside  lap  is 
the  steam  lap  and  the  outside  lap  is  the  exhaust  lap.  A  slide 
valve  arranged  for  inside  admission  is  illustrated  in  Fig.  81.  As 
shown  here  the  piston  is  at  the  head  end  of  its  stroke  and  steam 
is  being  admitted  to  the  head  end  of  the  cylinder.  The  valve 
is  therefore  displaced  to  the  left  of  its  mid-position  a  distance 
equal  to  the  steam  lap  plus  the  lead,  and  the  eccentric  must  be 
on  the  left-hand  side  of  the  vertical  line  AB  in  the  diagram  to  the 
right  of  Fig.  81.  In  order  for  the  engine  to  run  the  valve  must 
move  to  the  left  when  the  piston  starts  on  its  forward  stroke. 

If  this  valve  is  connected  directly  to  the  eccentric,  without  a 
rocker  arm,  and  the  rotation  is  to  be  clockwise  the  eccentric  must 
be  in  the  position  OE  when  the  crank  is  in  the  position  OC,  since 


142  STEAM  ENGINES 

in  this  position  the  valve  will  be  moved  to  the  left  and  be  opened 
a  greater  distance  when  the  crank  moves  in  a  clockwise  direction. 
If  the  crank  should  move  in  a  counterclockwise  direction  the  valve 
would  close  and  cut  off  the  supply  of  steam  when  the  piston  started 
forward.  Suppose  the  eccentric  was  placed  at  OEf  when  the 
crank  is  at  OC.  The  valve  would  then  be  in  the  position  shown 
in  Fig.  81  but  a  clockwise  rotation  of  the  shaft  would  move  the 
valve  to  the  right  and  close  the  port.  However,  if  the  shaft 
rotates  in  a  counterclockwise  direction  the  valve  would  move 
to  the  left  and  open  the  port  further  when  the  piston  starts 
forward.  It  may  be  stated,  then,  that  for  an  inside  admission 
valve  without  a  rocker  arm  the  eccentric  should  be  set  to  follow  the 
crank.  Since  a  rocker  arm  reverses  the  direction  of  motion  of 
the  valve,  the  presence  of  a  rocker  arm  requires  that  the  eccentric 
be  set  to  lead  the  crank,  that  is,  in  Fig.  81  if  there  were  a  rocker 
arm  between  the  eccentric  and  the  valve,  a  clockwise  direction  of 
rotation  would  require  that  the  eccentric  be  set  at  OE'  and  a 
counterclockwise  rotation  would  require  that  it  be  set  at  OE. 
In  Fig.  81  the  angle  of  advance  is  BOE  and  it  is  negative,  since 
the  crank  angle  is  less  than  90°.  The  angle  of  advance  for  a 
valve  with  inside  admission  does  not  differ  in  amount  from  that 
for  a  valve  with  outside  admission,  since  its  amount  depends 
only  upon  the  steam  lap  and  the  lead,  but  it  does  differ  in  position. 


. 
J 

•  tt 

CHAPTER  XI 
THE  VALVE  DIAGRAM 

Valve  Displacement. — Since  an  eccentric  is  equivalent  to  a 
crank,  it  may  be  represented  as  a  crank  having  a- length  equal 
to  the  eccentricity.  In  Fig.  82  the  valve  V  is  connected  by  the 
eccentric  rod  E  to  the  eccentric  OC,  the  eccentric  being  repre- 
sented here  by  a  crank  having  a  length  OC  equal  to  the  distance 
from  the  center  of  the  shaft  to  the  center  of  the  eccentric.  The 
circle  ABCF  represents  the  path  followed  by  the  center  of  the 
eccentric  as  the  shaft  rotates — and  its  diameter  AB  shows  the 
length  of  the  valve  travel,  which  is  twice  the  eccentricity  OC. 

When  the  center  of  the  eccentric  C  is  at  the  point  A,  the  valve 
is  at  the  extreme  left  of  its  travel,  when  C  is  at  the  point  B  the 


FIG.  82. 

valve  is  at  the  extreme  right  of  its  travel,  and  when  C  is  at  H 
the  valve  is  in  its  mid-position.  As  C  passes  through  the  half 
circle  AHB  the  valve  moves  through  a  distance  equal  to  that 
from  A  to  B,  and  as  C  passes  through  the  half  circle  BKA  the 
valve  moves  through  a  distance  equal  to  that  from  B  to  A .  The 
diameter  A  B  represents  the  valve  travel  to  the  same  scale  that 
OC  represents  the  eccentricity,  and  the  position  of  the  valve 
at  any  time  during  its  travel  may  be  located  on  the  diameter  AB. 
It  will  be  observed  that  the  eccentric  rod  is  long  in  comparison 
with  the  valve  travel,  and  that  during  a  revolution  of  the  shaft 
the  eccentric  rod  never  makes  a  large  angle  with  the  center  line 
BL.  When  this  is  the  case  the  position  of  the  valve  may  be 
located  for  any  position  of  the  eccentric  by  simply  projecting 
vertically  to  the  diameter  AB,  the  point  representing  the  center 
of  the  eccentric.  Thus,  when  the  eccentric  is  in  the  position 

143 


144 


STEAM  ENGINES 


/ 


«r~-~";r'* 


\\ 


"Vf-      t— 


OC  the  valve  will  be  at  the  point  D  in  its  travel,  the  point  D  being 
tt  found  by  drawing  CD  at  right  angles 

to  AB.  The  distance  OD  is  the  dis- 
placement  of  the  valve  from  its  mid- 
position.  When  the  eccentric  is  at 
OF,  G  represents  the  position  of  the 
valve,  and  OG  its  displacement  from 
mid-position.  It  will  be  observed 
that  if  F  is  exactly  opposite  C,  the 
valve  displacement  OG  when  the 
eccentric  is  •  at  OF  is  the  same  as  its 
displacement  OD  when  the  eccentric 
is  at  OC. 

Piston  Position.  —  The  same  kind  of 
diagram  as  shown  in  Fig.  82  may  be 

%\  used  to  represent  the  travel  of  the 

piston,  but  the  method  of  locating 
the  position  of  the  piston  differs  on 
account  of  the  fact  that  the  connect- 
ing rod  is  usually  shorter  when  com- 
pared to  the  length  of  the  piston  travel 
and  that  at  certain  parts  of  the  revo- 
lution of  the  crank,  the  connecting 
rod  makes  a  considerable  angle  with 
the  center  line  of  the  engine. 

In  Fig.  83  the  circle  AHBK  repre- 
sents the  path  followed  by  the  center 
of  the  crank  pin  C  during  a  revolution 
of  the  shaft  whose  center  is  at  0.  OC 
is  the  length  of  the  crank  and  the 
diameter  AB  of  the  crank  circle  rep- 
resents the  piston  stroke.  When  the 
crank  is  in  any  position  as  OC,  the 
corresponding  position  of  the  piston 
may  be  located  by  taking  a  radius 
equal  to  the  length  of  the  connecting 
rod  MC,  and  with  M  as  a  center 
drawing  an  arc  CD  through  C  until 
it  strikes  the  diameter  A  B  at  the 
point  D.  The  point  D  will  represent 
the  position  of  the  piston  for  the  posi- 


THE  VALVE  DIAGRAM  145 

tion  OC  of  the  crank  and  the  distance  AD  shows  how  far  the 
piston  is  from  the  end  of*  its  stroke  when  the  crank  is  at  OC. 
The  apparent  position  of  the  piston  is  at  R,  obtained  by  pro- 
jecting the  point  C  vertically  on  the  diameter  AB  as  was  done 
for  the  valve  position  in  Fig.  82.  The  actual  position  D  of  the 
piston  is  displaced  from  its  apparent  position  R  by  the  distance 
RD  and  this  is  due  to  the  comparatively  large  angle  which  the 
connecting  rod  makes  with  the  center  line  LB  of  the  engine. 
When  the  crank  is  in  the  position  OF,  exactly  opposite  OC,  the 
actual  position  of  the  piston  is  at  G  while  its  apparent  position 
is  at  S.  This  effect  is  due  to  the  angularity  of  the  connecting  rod 
and  its  amount  depends  upon  the  length  of  the  connecting  rod 


FIG.  84. 

as  compared  with  the  length  of  piston  stroke.  In  Corliss  engines 
the  length  of  connecting  rod  is  so  great,  as  compared  with  the 
piston  stroke,  that  the  angularity  of  the  connecting  rod  seldom 
need  be  taken  into  account.  Other  types  of  engines,  on  the 
other  hand,  have  comparatively  short  connecting  rods  and  the 
angularity  must  be  considered  in  locating  the  position  of  the 
piston  for  cut-off,  release,  and  compression,  and  sometimes  also 
for  admission,  which  occurs  nearer  the  end  of  the  stroke  than  any 
of  the  other  events  and  hence  is  not  so  much  affected  by  the 
angularity  of  the  connecting  rod. 

Position  of  Crank  and  Eccentric. — In  Fig.  82  it  is  shown  that 
the  position  of  the  eccentric  and  the  valve  displacement  may  be 
represented  on  a  circle  having  a  radius  equal  to  the  eccentricity 


146  STEAM  ENGINES 

and  in  Fig.  83  it  is  shown  that  the  position  of  the  crank  and  piston 
may  be  represented  on  a  circle  having  a  radius  equal  to  the  length 
of  the  crank.  The  corresponding  positions  of  crank  and  eccentric 
may  therefore  be  represented  by  two  circles  drawn  about  the 
same  center,  one  for  the  crank  and  one  for  the  eccentric,  as  shown 
in  Fig.  84. 

In  most  engines  the  eccentricity  is  small  as  compared  with  the 
length  of  the  crank,  hence  if  both  the  crank  circle  and  the  eccen- 
tric circle  are  drawn  to  the  same  scale,  one  will  be  large  and  the 
other  small,  and  it  may  happen  that  the  eccentric  circle  will  be  so 
small  as  to  cause  difficulty  in  making  measurements  upon  it. 
In  Fig.  84  the  length  of  the  crank  OC  is  12  inches,  giving  a  stroke 
of  24  inches,  and  the  eccentricity  OE  is  2 
inches,  giving  a  valve  travel  of  4  inches, 
these  being  common  proportions  between 
valve  travel  and  stroke.  In  order  to  draw 
the  corresponding  positions  of  crank  and 
eccentric  upon  these  circles,  the  position 
of  the  crank  OC  is  first  drawn.  Then  by 
FIG  85  laying  off  the  crank  angle  COE,  the  eccentric 

OE  may  be  drawn. 

It  will  be  seen  from  Fig.  84  that  if  both  the  crank  circle  and 
the  eccentric  circle  be  drawn  to  the  same  scale  it  will  be  difficult 
to  measure  valve  displacements  accurately  on  account  of  the 
small  size  of  the  eccentric  circle.  In  order  to  avoid  this  difficulty 
the  eccentric  circle  may  be  drawn  the  same  size  as  the  crank 
circle,  thus  making  a  single  circle  serve  for  both  crank  circle  and 
eccentric  circle,  as  in  Fig.  85.  If  this  is  done  the  diameter  of  the 
circle  will  represent  the  piston  stroke  to  one  scale  and  the  valve 
travel  to  a  different  scale.  Thus,  in  Fig.  85  the  diameter  AB 
represents  a  piston  stroke  of  24  inches  and  it  also  represents  a 
valve  travel  of  4  inches.  For  the  crank  position  OC  the  piston 
is  at  a  distance  AD  from  the  end  of  its  stroke  and  this  distance 
is  3.6  inches  on  the  scale  by  which  the  diameter  represents  24 
inches.  The  valve  displacement  corresponding  to  the  crank 
position  OC  is  shown  at  OF  and  this  distance  is  1.6  inches  on  the 
scale  by  which  the  diameter  represents  4  inches. 

Valve  Diagram. — A  diagram  may  be  drawn  which  shows  the 
valve  displacement  for  all  positions  of  the  crank.  Such  a  dia- 
gram is  called  a  valve  diagram.  A  valve  diagram  is  useful  in 
setting  the  valve  because  it  shews  at  a  glance  the  effects  of  any 


THE  VALVE  DIAGRAM  147 

changes  which  may  be  made  in  the  valve  or  eccentric  and  thus 
tells  what  changes  to  makedn  order  to  accomplish  desired  results. 
The  diagram  shown  in  Fig.  &5  might  be  used  as  a  valve  diagram 
'since  from  it  could  be  obtained  the  valve  displacement  for  any 
position  of  the  crank,  but  it  is  not  convenient  to  use  this  dia- 
gram because  the  crank  angle  must  be  laid  off  for  each  new 
position  of  the  crank  at  which  it  is  desired  to  measure  the  valve 
displacement. 

The  most  common  form  of  valve  diagram  is  called  the  Zeuner 
diagram,  after  the  name  of  its  inventor.  The  Zeuner  valve 
diagram  is  drawn  as  follows:  The  circle  ACS  in  Fig.  86  is 
drawn  so  that  its  diameter  represents  the  piston  stroke  to  one 
scale  and  the  valve  travel  to  another  scale.  Imagine  the  eccen- 
tric to  be  in  the  position  OB ;  the  crank  will 
then  be  in  the  position  OC  and  the  angle 
COB  will  represent  the  crank  angle.  With 
the  eccentric  in  the  position  OB  the  valve 
will  be  at  the  extreme  right  of  its  travel 
and  its  displacement  from  mid-position 
will  be  a  maximum,  being  equal  to  the 
eccentricity.  Using  the  line  OC,  which 
represents  the  crank,  as  a  diameter,  draw 

the  small  circle  OPC.  This  is  called  the  valve  circle.  Any  radial 
line  drawn  from  0  to  represent  any  position  of  the  crank,  and 
cutting  the  valve  circle,  will  show  the  valve  displacement  from 
mid-position  by  the  length  which  the  valve  circle  cuts  off  on 
it.  Thus,  when  the  crank  is  in  the  position  OC,  the  valve  dis- 
placement is  equal  to  the  length  OC.  When  the  crank  is  in  any 
other  position,  as  at  ON  the  valve  displacement  is  OP,  being 
always  the  length  which  the  valve  circle  cuts  off  on  the  radial 
line  which  represents  the  crank  position. 

It  is  to  be  observed  that  in  Fig.  86  the  angle  COE  represents 
the  angle  of  advance  since  the  angle  of  advance  is  equal  to  the 
crank  angle  minus  90°.  In  this  case  the  angle  COB  is  the  crank 
angle  and  taking  away  90°,  or  the  angle  EOB,  leaves  the  angle 
COE  as  the  angle  of  advance. 

In  the  preceding  chapter  it  was  shown  that  the  valve  displace- 
ment at  admission  and  cut-off,  is  equal  to  the  steam  lap.  There- 
fore, if  the  arc  of  a  circle  be  drawn  with  O,  Fig.  87,  as  a  center  and 
a  radius  OL  equal  to  the  steam  lap,  it  will  cut  the  valve  circle  at 
the  points  H  and  J  and  a  line  OD  drawn  through  0  and  H  will 


148 


STEAM  ENGINES 


represent  the  position  of  the  crank  at  admission,  when  the  valve 
displacement  is  equal  to  OH,  the  steam  lap.  •  Also,  a  line  OG 
drawn  through  0  and  J  will  represent  the  position  of  the  crank  at 
cut-off  when  the  valve  displacement  is  again  equal  to  the  steam 
lap,  OJ.  The  arc  HLJ  is  called  the  steam  lap  circle.  Fig.  87 
is  drawn  in  the  same  way  as  Fig.  86,  but  is  made  separate  in 
order  to  avoid  confusion. 

The  lead  of  a  valve  is  the  amount  of  port  opening  when  the 
crank  is  on  dead  center.  The  port  opening  is  equal  to  the  valve 
displacement  minus  the  steam  lap.  In  Fig.  87,  OA  represents 
the  position  of  the  crank  when  on  dead  center.  OM  shows  the 


valve  displacement  for  this  position  of  the  crank  and  OL  the 
steam  lap,  hence  LM  represents  the  lead  of  the  valve.  The 
amount  of  port  opening  is  always  the  distance  between  the  lap 
circle,  and  the  valve  circle,  as  shown  by  the  shaded  area  in  Fig 
87. 

It  should  be  observed  that  the  diagram  in  Fig.  87  shows  only 
valve  displacements  to  the  right  of  mid-position  and  for  this 
reason  the  valve  circle  is  marked  R.  In  order  to  show  valve 
displacements  to  the  left  of  mid-position  another  valve  circle 
L'  must  be  drawn  opposite  the  one  marked  R,  as  shown  in  Fig. 
88.  The  valve  circle  marked  L'  for  showing  displacements  to  the 
left  must  be  the  same  size  as  the  other  one  and  must  be  drawn 
on  the  same  line  OC  extended  through  the  crank  circle.  Since 
release  and  compression  occur  when  the  valve  displacement  is 


THE  VALVE  DIAGRAM 


149 


equal  to  the  exhaust  lap,  the  lap  circle  TS  must  be  drawn  with  a 
radius  equal  to  the  exhaust^  la"p.  The  four  positions  of  the  crank, 
at  admission,  cut-off,  release,  and  compression  may  now  be 
drawn  on  the  diagram  as  shown  in  Fig.  88.  Since  admission  and 
cut-off,  as  shown  here,  take  place  when  the  valve  is  displaced  to 
the  right,  and  release  and  compression  take  place  when  the  valve 
is  displaced  to  the  left,  these  events  are  all  for  one  end  of  the 
cylinder  only.  In  Fig.  88  the  events  shown  are  for  the  head  end 


CUT-OFF 


ADMISSION  I 


HEAD -END    INDICATOR  DIAGRAM 

FIG.  88. 

of  the  cylinder,  if  there  is  no  rocker  arm,  the  direction  of  rotation 
being  clockwise,  as  indicated  by  the  arrowhead. 

With  the  crank  positions  at  admission,  cut-off,  release,  and 
compression  as  shown  in  Fig.  88  the  approximate  shape  of  the 
indicator  diagram  that  will  be  obtained  from  this  valve  setting 
may  be  shown  if  the  admission  and  exhaust  pressures  are  assumed. 
In  order  to  draw  this  indicator  diagram  the  crank  positions  are 
projected  to  the  line  AB  with  a  radius  equal  to  the  length  of  the 
connecting  rod  (to  the  same  scale  that  AB  represents  the  piston 

15 


150  STEAM  ENGINES 

stroke) .  These  points  are  then  projected  vertically  to  the  admis- 
sion and  exhaust  lines  below  and  the  indicator  diagram  sketched 
in  as  shown,  using  smooth  curves  to  connect  cut-off  and  release, 
and  also  compression  and  admission. 

The  effects  of  certain  changes  in  the  valve  setting  may  readily 
be  observed  from  the  valve  diagram  in  Fig.  88.  For  a  given 
eccentric  the  eccentricity  is  a  fixed  quantity  and  cannot  be 
changed.  The  steam  and  exhaust  laps  are  parts  of  the  valve  and 
cannot  be  readily  changed  although  they  may  be  decreased 
slightly  by  chipping  or  filing  off  the  end  of  the  valve.  This 
leaves  only  the  angle  of  advance  that  may  be  changed  readily, 
which  may  be  done  by  shifting  the  eccentric  around  on  the  shaft 
either  to  increase  the  angle  of  advance  or  to  decrea'se  it. 

The  angle  COE  in  Fig.  88  represents  the  angle  of  advance.  It 
will  be  observed  from  the  valve  diagram  that  if  the  angle  of 
advance  is  made  larger  admission,  cut-off,  release,  and  compres- 
sion will  all  occur  earlier  in  the  stroke.  If  the  angle  of  advance 
is  made  smaller  all  of  these  events  will  occur  later  in  the  stroke. 
It  will  be  observed  also  that,  other  things  being  left  unchanged, 
a  large  steam  lap  gives  late  admission  and  early  cut-off  and  a  large 
exhaust  lap  gives  late  release  and  early  compression.  The  steam 
and  exhaust  laps  might  be  increased  by  fastening  a  block  to  the 
outer  or  inner  edges  of  the  valve  but  this  is  not  often  done  as  it  is 
inconvenient  and  not  often  necessary  since  the  valve  is  designed 
with  the  proper  laps.  A  smaller  steam  lap  causes  early  admission 
and  late  cut-off  and  a  small  exhaust  lap  causes  early  release  and 
late  compression.  As  explained  before,  the  steam  and  exhaust 
laps  may  be  made  slightly  smaller  by  filing  or  chipping,  but  this 
is  not  often  necessary. 

It  will  be  further  observed  from  the  valve  diagram  that  the 
angle  which  the  crank  turns  through  while  the  steam  is  being  com- 
pressed in  the  cylinder,  is  the  same  as  the  angle  turned  through 
while  the  steam  is  expanding.  This  sets  an  important  limitation 
upon  the  action  of  the  slide  valve  because  it  does  not  permit 
so  large  a  degree  of  expansion  of  the  steam  with  economy  as 
might  otherwise  be  secured.  In  order  to  secure  good  economy 
an  engine  must  expand  the  steam  through  a  large  range  of  pres- 
sure. The  range  of  pressure  through  which  steam  may  be  ex  panded 
depends  upon  the  point  of  cut-off;  if  cut-off  is  early  the  range 
of  pressure  will  be  large,  but  if  cut-off  is  late  the  range  of  pressure 
will  be  small.  If  it  is  attempted  to  secure  an  early  cut-off 


THE  v  VALVE  DIAGRAM 


151 


with  a  slide  valve  the  point  of  compression  will  also  be  early. 
If  the  cut-off  is  made  earlier  than  about  half  stroke,  the  gain  from 
greater  expansion  will  be  mgre  than  counterbalanced  by  the  loss 
from  greater  compression.  This  result  may  be  seen  from  Fig. 
89  in  which  the  full  line  diagram  shows  cut-off  at  seven-eighths 
of  the  stroke  with  the  corresponding  compression,  and  the  dotted 
lines  show  the  greater  expansion  for  cut-off  at  half  stroke  with  the 
corresponding  compression.  With  the  cut-off  at  half  stroke  the 
gain  in  economy  from  the  greater  expansion  is  counterbalanced 
by  the  loss  of  area  from  the  indicator  diagram  by  the  earlier 
compression.  The  action  of  the  slide  valve  described  above  ex- 
plains why  an  engine  fitted  with  this  type  of  valve  is  uneconom- 


FIG.  89. 

ical  in  the  use  of  steam;  that  is,  it  cannot  use  the  full  expansive 
force  of  the  steam  as  can  engines  which  have  separate  valves  for 
admission  and  exhaust. 

The  valve  diagram  in  Fig.  88  shows  the  events  occurring  in 
only  one  end  of  the  cylinder,  namely,  the  head  end.  The  same 
diagram  may  also  be  used  for  showing  events  occurring  in  the 
crank  end  of  the  cylinder  by  drawing  the  crank  end  steam  lap 
circle  in  the  left  (I/)  valve  circle  and  the  crank  end  exhaust  lap 
circle  in  the  right  (R)  valve  circle.  This  has  been  done  in  Fig. 
90,  the  steam  and  exhaust  lap  circles  for  the  head  end  being 
drawn  with  full  lines  and  the  steam  and  exhaust  lap  circles  for 
the  crank  end  being  dotted.  The  crank  positions  for  events  in 
the  head  end  are  also  drawn  with  full  lines  and  those  for  the 
crank  end  with  dotted  lines.  This  makes  a  complete  valve 
diagram  which  shows  all  of  the  actions  occurring  in  both  ends 
of  the  cylinder  and  shows  also  the  effects  produced  by  any  changes 
in  the  valve  or  its  setting. 


152 


STEAM  ENGINES 


All  of  the  valve  diagrams  shown  up  to  the  present  time  have 
been  for  a  clockwise  direction  of  rotation  and  for  a  direct  con- 


CVJT-  OFF 
H      E.ND 

MPRESSION 
C.-E.ND 


ADMISSION 

H    END     - 


RELEASE 
C    END 


RELEASE. 
H.  END 


^-\  ADMISSION 
C.-END 


COMPRESSION 
H.END 
CUT-  OFF 

C,E~ND 


FlQ.    90. 


nection  between  valve  and  eccentric.     Fig.  91  shows  how  the 
valve  diagram  should  be  drawn  for  a  counterclockwise  direction 


CUT-OFF 
C-  END 


COMPRESSION 
H.  END 


RELEASE. 
C.  END  " 

ADMISSION 
H.  END 


ADMISSION 
C.ENO 


RE.LEA3E. 
'    H.E.ND 


COMPRESSION 
~    C.  END 


FlQ.    91. 


of  rotation.     In  this  case  the  angle  of  advance  COE  is  laid  off 
to  the  right  of  the  vertical  line  EF  instead  of  to  the  left  of  it  as 


THE  VALVE  DIAGRAM  153 

with  clockwise  rotation;  and  in  all  cases  the  valve  circles  are 
drawn  on  the  line  representing  the  position  of  the  crank  when  the 
eccentric  is  on  dead  center^.  In  Fig.  91  the  crank  will  be  at 
OC  when  the  eccentric  is  at  OA ,  hence  the  valve  circles  are  drawn 
on  the  diameter  through  C  and  0.  Valve  displacements  to  the 
right  of  mid-position  are  then  measured  on  the  bottom  valve 
circle  R  and  those  to  the  left  are  measured  on  the  top  valve  circle 
U .  Admission  and  cut-off  for  the  head  end  are  therefore  located 
by  means  of  the  bottom  valve  circle  and  release  and  compression 
for  the  head  end  by  means  of  the  top  valve  circle.  Events  for 
the  crank  end  of  the  cylinder  are  located  by  means  of  the  opposite 
valve  circles  from  those  for  the  head  end  events. 

The  late  cut-off  generally  employed  •  with  plain  slide  valve 
engines  makes  them  uneconomical  on  account  of  not  using  much 


FIG.  91a. 

of  the  expansive  force  of  the  steam.  With  a  late  cut-off  the 
terminal  pressure,  or  pressure  at  the  end  of  expansion  will  be 
high.  When  the  exhaust  port  is  opened,  this  high  pressure 
remaining  in  the  steam  is  wasted  through  the  exhaust  pipe. 

When  such  an  engine  is  operated  under  a  fairly  constant  load 
and  not  stopped  at  frequent  intervals  the  steam  consumption 
may  sometimes  be  greatly  reduced  by  redesigning  its  valve, 
changing  its  steam  and  exhaust  taps,  and  re-setting  the  eccentric. 
Fig.  91a  illustrates  a  set  of  indicator  diagrams  from  an  engine 
whose  valve  was  redesigned  to  give  an  earlier  cut-off.  It  can 
be  seen  from  these  diagrams  that  the  steam  distribution  and 
economy  is  much  better  than  is  obtained  with  the  ordinary  slide 
valve  cutting  off  at  about  three-quarters  stroke.  This  valve  was 
redesigned  so  as  to  give  cut-off  at  about  half  stroke,  release  at 
about  90  per  cent,  of  the  stroke,  and  compression  at  about  72 
per  cent,  of  the  exhaust  stroke.  The  angle  of  advance  was  also 
changed  to  about  50°.  Making  the  cut-off  earlier  increases  the 


154  STEAM  ENGINES 

probability  that  the  engine  will  stop  in  a  position  after  the  valve 
has  closed  against  admission  which  makes  it  impossible  to  start 
again  until  the  flywheel  has  been  turned,  but  if  the  engine  is 
seldom  stopped  during  the  day,  this  is  not  likely  to  become  a 
nuisance. 

Defects  in  the  setting  or  adjustment  of  valves  are  readily  de- 
tected by  irregularities  in  the  indicator  diagram.  A  study  of  the 
valve  diagram  will  show  the  causes  of  such  defects  in  the  valve  set- 
ting and  will  suggest  the  remedy  that  should  be  applied.  One  of 
the  most  common  defects  in  valve  adjustment  comes  from  slipping 
of  the  eccentric  around  on  the  shaft,  the  eccentric  usually  being 
fastened  to  the  shaft  by  a  single  set  screw.  If  the  eccentric 
slips  it  is  likely  to  do  so  against  the  direction  of  rotation,  resulting 
in  a  decrease  of  the  angle  of  advance.  A  decrease  in  the  angle 


FIG.  92. 

of  advance  causes  all  of  the  events  (admission,  cut-off,  release,  and 
compression)  to  occur  later.  The  effect  on  the  indicator  diagram 
is  shown  in  Fig.  92,  which  was  taken  from  an  engine  on  which  the 
eccentric  had  slipped  backward.  The  most  noticeable  defect 
due  to  the  slipping  backward  of  the  eccentric  is  seen  at  release 
where  the  diagram  will  have  a  beak,  as  at  1,  2,  showing  that  the 
valve  does  not  open  soon  enough  to  allow  the  pressure  in  the 
cylinder  to  fall  to  the  exhaust  pressure  before  the  piston  starts  on 
the  return  stroke.  Slippage  of  the  eccentric  cannot  be  detected 
readily  from  the  late  cut-off  because  some  engines  have  a  much 
later  cut-off  than  others.  With  a  properly  designed  slide  valve, 
however,  late  release  is  always  accompanied  by  late  cut-off.  If 
the  valve  has  proper  lead  before  the  eccentric  slips  backward, 
it  will  have  too  little  lead  afterwards.  The  effects  of  this  on  the 
admission  line  is  shown  by  the  rounding  from  4  to  5  caused  by  the 
admission  side  of  the  valve  opening  too  late,  thus  preventing  the 
pressure  in  the  cylinder  from  rising  quickly  and  also  causing 


THE  VALVE  DIAGRAM 


155 


wire-drawing.  Small  lead  is  not  a  good  indication  of  a  slipped 
eccentric  because  the  valve  may  have  been  set  originally  with 
too  little  lead.  The  compression,  in  this  case,  is  too  late  for  a 
slide  valve  and  gives  another  indication  that  the  eccentric  has 
slipped  backward. 

If,  by  any  means,  the  eccentric  should  become  turned  forward 


FIG.  93. 

on  the  shaft,  the  angle  of  advance  will  become  larger,  and  all  of 
the  events  will  occur  earlier.  An  indicator  diagram  showing  the 
results  of  too  great  angle  of  advance  is  illustrated  in  Fig.  93. 
Too  early  release  is  shown  at  3,  4,  by  the  sharp  toe  of  the  diagram, 
Early  admission,  the  result  of  too  much  lead,  is  indicated  at  6. 


Head  End 


FIG.  93a. 

1,  by  the  backward  pointing  beak.  The  cut-off,  2,  is  evidently 
too  early  for  a  slide  valve  engine,  although,  in  gejieral,  the 
position  of  the  cut-off  does  not  give  a  good  indication  of  the 
valve  setting.  The  early  compression,  5,  agrees  with  the  early 
cut-off,  as  it  should  with  a  slide  valve. 

Sometimes  the  valve  becomes  displaced  on  the  valye  stem, 


156  STEAM  ENGINES 

resulting  in  the  valve  stem  being  either  too  long  or  too  short 
for  the  valve  setting  desired.  This  will  affect  the  events  oc- 
curring in  the  two  ends  of  the  cylinder  differently  since  the 
'  valve  is  moved  bodily  either  to  the  right  or  to  the  left.  If  the 
valve  stem  is  too  long  the  steam  lap  on  the  head  end  is  increased, 
which  delays  admission  and  hastens  cut-off  on  the  head  end,  as  il- 
lustrated in  Fig.  93a.  The  valve  stem  being  too  long  also  de- 
creases the  exhaust  lap  on  the  head  end,  which  hastens  release  and 
delays  compression  on  the  head  end.  The  effects  produced  on  the 
events  of  the  crank  end  as  illustrated  in  Fig.  936  are  just  opposite 


Crank  End 


FIG.  936. 

from  those  produced  on  the  head  end.  The  steam  lap  on  the 
crank  end  is  reduced,  which  hastens  admission  and  delays  cut-off, 
while  the  crank  end  exhaust  lap  is  increased,  which  delays  release 
and  hastens  compression. 

The  changes  in  events  produced  by  shortening  the  valve  stem 
will  be  the  same  as  those  produced  by  lengthening  it,  except  that 
the  changes  produced  in  the  head  end  events  by  lengthening  the 
valve  stem  will  be  produced  in  crank  end  events  if  the  valve  stem 
is  shortened,  and  the  changes  produced  in  the  crank  end  events 
by  lengthening  will  be  produced  in  the  head  end  if  the  valve  stem 
is  shortened. 


CHAPTER  XII 
VALVE  SETTING 

General  Considerations. — In  setting  the  valves  of  an  engine  the 
principal  requirements  are  to  secure  an  economical  distribution 
of  steam  between  the  two  ends  of  the  cylinder  and  to  secure 
smooth  running  of  the  engine.  In  order  to  obtain  these  results 
it  is  desirable  to  divide  the  work  to  be  performed  equally  between 
the  two  ends  of  the  cylinder.  An  unequal  division  of  the  work 
between  the  two  ends  of  the  cylinder  throws  unduly  large  strains 
upon  the  engine,  is  likely  to  cause  variations  in  the  speed,  and 
causes  the  steam  to  be  used  under  more  unfavorable  conditions 
in  one  end  of  the  cylinder  than  in  the  other.  An  equal  division 
of  the  work  will  be  secured  approximately  when  cut-off  occurs 
at  equal  points  of  the  admission  strokes  for  the  two  ends  of  the 
cylinder,  therefore,  in  setting  engine  valves  it  is  desirable  to  secure 
equal  cut-offs  for  the  two  ends  of  the  cylinder. 

Vertical  engines  form  an  exception  to  the  above  statements 
as  in  these,  the  cut-off  in  the  crank  end  of  the  cylinder  should 
occur  later  than  that  in  the  head  end  by  enough  to  lift  the  weight 
of  the  piston,  piston  rod,  crosshead,  and  connecting  rod. 

Another  requirement  towards  securing  smooth  running  is  that 
there  should  be  equal  leads  on  the  two  ends  of  the  cylinder  as, 
by  this  means,  the  cushioning  effect  at  each  end  of  the  piston 
stroke  is  made  equal  and,  if  the  lead  is  sufficient  for  the  size  and 
speed  of  the  engine,  it  will  pass  the  dead  centers  smoothly  and 
without  shock. 

It  appears  from  the  above  discussion  that  engine  valves  should 
be  set  with  respect  to  the  cut-off  and  lead.  An  inspection  of 
Fig.  90  will  show  that  both  the  cut-off  and  lead  depend  upon  the 
eccentricity,  angle  of  advance,  and  steam  lap.  All  of  these, 
except  the  angle  of  advance,  are  fixed  dimensions  and  cannot  be 
readily  changed  after  the  engine  is  built.  However,  as  valve 
gears  are  usually  constructed,  the  length  of  the  valve  stem  may 
be  changed  or,  what  amounts  to  the  same  thing,  the  position  of 
the  valve  on  the  stem  may  be  changed  Changing  the  length 
16  157 


158 


STEAM  ENGINES 


of  the  valve  stem,  which  moves  the  valve  bodily  along  on  its 
seat,  has  the  effect  of  increasing  the  steam  lap  and  decreasing  the 
exhaust  lap  on  one  end,  and  decreasing  the  steam  lap  and  in- 
creasing the  exhaust  lap  on  the  other  end.  For  example,  if  the 
length  of  the  valve  stem  is  increased,  the  steam  lap  on  the  head 
end  will  be  increased,  the  exhaust  lap  on  this  end  decreased  and 
the  steam  lap  on  the  crank  end  will  be  decreased  and  the  exhaust 
lap  on  this  end  increased.  In  setting  valves,  therefore,  the  two 
things  that  may  be  changed  are  the  angle  of  advance  and  the 
length  of  the  valve  stem. 


CUT-OFF 


FIG.  94. 

An  inspection  of  Fig.  94  will  show  that  if  a  valve  has  equal 
steam  laps  on  head  and  crank  ends,  the  leads  on  the  two  ends  will 
also  be  equal.  If  the  connecting  rod  was  long  enough  so  that 
its  angularity  did  not  affect  the  motion  of  the  piston  the  cut-off 
on  the  two  ends  would  also  be  equal  when  the  steam  laps  are 
equal  but  actually,  the  angularity  of  the  connecting  rod  does  af- 
fect the  motion  of  the  piston  and  causes  cut-off  to  occur  later  in 
the  stroke  from  head  to  crank  end  than  it  does  in  the  stroke  from 
crank  to  head  end.  It  may  be  stated  as  a  general  rule,  therefore, 
that  an  ordinary  slide  valve  cannot  be  set  to  give  both  equal  cut-offs 


VALVE  SETTING  159 

and  equal  leads,  although  it^  would  be  desirable  if  this  could  be 
done. 

If  the  valve  gear  contains  a  rocker  arm  which  reverses  the 
motion  of  the  valve  it  is  possible  to  shape  the  rocker  arm  so  the 
angularity  of  the  connecting  rod  may  be  compensated  for  and  the 
cut-offs  made  equal  while  retaining  equal  leads,  but  this  can  be 
done  for  cut-off  at  only  one  point  of  the  stroke.  If  the  valve  is 
set  to  cut  off  at  any  other  part  of  the  stroke  either  the  cut-offs 
or  the  leads  will  be  unequal.  Rocker  arms  of  this  kind  are  not 
made  straight,  but  instead,  the  part  on  one  side  of  the  pivot  is 
made  at  an  angle  to  the  part  on  the  other  side  of  the  pivot. 
These  are  sometimes  used  on  automatic  high  speed  engines  but 
are  seldom  used  on  plain  slide  valve  engines. 

A  plain  slide  valve  may  be  set  to  give  equal  leads  on  the  two 
ends  of  the  cylinder,  the  cut-offs  being  unequal,  or  to  give  equal 
cut-offs,  the  leads  being  unequal,  or  a  compromise  may  be  made 


FIG.  95. 

and  the  valve  set  to  give  slightly  unequal  leads  and  slightly 
unequal  cut-offs. 

In  setting  engine  valves  it  is  necessary  to  place  the  engine  on 
dead  center.  A  person  cannot  judge  when  an  engine  is  exactly 
on  dead  center  because  when  near  the  end  of  the  stroke  the  crank 
moves  through  a  considerable  angle  while  the  piston  and  cross- 
head  move  very  little  For  this  reason  it  is  desirable  to  use  some 
method  for  putting  the  engine  exactly  on  dead  center  and  the 
following  method  will  be  found  satisfactory. 

Placing  an  Engine  on  Center. — An  engine  is  placed  on  center 
by  means  of  a  tram}  which  is  an  iron  rod  pointed  at  both  ends 
and  having  one  end  bent  at  right  angles  to  the  main  part  of  the 
rod,  as  shown  in  Fig.  95.  A  tram  about  30  inches  long  and  made 
of  a  rod  %Q  inch  in  diameter  is  a  convenient  size  to  use. 

In  placing  an  engine  on  center  by  the  use  of  a  tram  the  fly- 
wheel is  turned  by  hand  until  the  crosshead  is  within  three  or  four 
inches  of  the  end  of  its  stroke.  In  turning  the  engine  by  hand  it 
should  always  be  turned  in  the  direction  in  which  it  ordinarily 
runs  in  order  to  take  up  the  lost  motion  or  backlash  in  the  various 


160 


STEAM  ENGINES 


bearings.  When  the  crosshead  has  been  brought  within  three 
or  four  inches  of  the  end  of  its  stroke  a  scratch  mark  is  made  on 
both  crosshead  and  guide,  as  shown  at  B  Fig.  96,  so  that  by 
bringing  the  parts  of  this  mark  together  at  any  time  the  crosshead 
will  be  in  the  same  position  as  before.  The  straight  end  of  the 
tram  is  now  placed  on  a  fixed  mark  on  the  floor  and  a  mark  is 


FIG.  96. 

made  with  bent  end  on  the  rim  of  the  flywheel  as  shown  at  A 
in  Fig.  96,  the  crosshead  still  being  in  such  position  that  the 
marks  on  crosshead  and  guide  are  together.  The  flywheel 
is  now  turned  by  hand  until  the  crank  is  past  center  and  the  cross- 
head  has  been  brought  to  the  same  position  as  before,  as  indicated 
by  the  marks  on  crosshead  and  guide  coinciding.  The  crank 


FIG.  97. 

will  now  be  as  far  past  center  as  it  was  in  front  of  center  before. 
With  the  engine  in  this  position  the  tram  is  placed  on  the  perma- 
nent mark  on  the  floor  and  another  mark  made  on  the  rim  of  the 
flywheel,  as  shown  at  C  in  Fig.  97.  With  a  tape  measure  or 
pair  of  dividers  find  the  point  X  midway  between  A  and  C.  Make 
a  center  punch  mark  here  and  turn  the  engine  until  the  tram, 
still  on  the  permanent  mark  on  the  floor,  falls  square  into  the 
punch  mark,  when  the  engine  will  be  exactly  on  center. 


VALVE  SETTING  161 

The  above  method  should  be  used  in  finding  the  other  dead 
center  position  and,  with  these  two  positions  marked,  the  engine 
may  be  quickly  placed  on  center  at  any  time  by  turning  it  until 
the  tram,  resting  on  the  permanent  mark  on  the  floor,  comes  to 
the  punch  mark  representing  the  dead  center  position. 

To  Set  Valves  With  Equal  Leads. — In  setting  the  valve,  two 
results  must  be  accomplished.  First,  the  valve  must  be  made  to 
travel  equal  distances  each  side  of  its  central  position,  thus  giving 
the  valve  equal  leads  on  its  two  sides;  second,  after  making  the 
leads  equal,  they  must  be  adjusted  to  the  desired  amount. 

In  order  to  accomplish  these  results,  two  adjustments  are 
possible;  first,  the  position  of  the  valve  on  the  rod  may  be 
changed,  or,  what  amounts  to  the  same  thing,  the  length  of  the 
valve  rod  may  be  changed;  second,  the  eccentric  may  be  shifted 
around  on  the  shaft.  Changing  the  length  of  the  valve  rod  (or 
the  position  of  the  valve  on  the  rod)  increases  the  lead  at  one  end 
of  the  valve  and  decreases  it  at  the  other  end.  If  the  rod  is  length- 
ened the  lead  will  be  increased  at  the  crank  end  and  decreased 
at  the  head  end.  If  it  is  shortened,  the  lead  will  be  increased 
at  the  head  end  and  decreased  at  the  crank  end.  Shifting  the 
eccentric  around  on  the  shaft  increases  both  leads  or  decreases 
both  leads,  depending  upon  which  direction  the  eccentric  is 
shifted.  If  the  eccentric  is  shifted  so  as  to  decrease  the  angle 
of  advance,  both  leads  are  shortened  and  if  the  angle  of  advance  is 
increased  both  leads  are  lengthened. 

In  setting  a  slide  valve,  proceed  as  follows:  (1)  Set  the  engine 
on  head  end  dead  center  and  (having  removed  the  cover  of  the 
valve  chest)  measure  the  lead  which  the  valve  has  on  that  end. 
If  the  valve  covers  the  port  (negative  lead)  mark  the  position 
of  the  valve  and  then  turn  the  engine  forward  until  the  edge 
of  the  valve  is  in  line  with  the  edge  of  the  port,  and  measure  the 
distance  which  the  port  was  overlapped  by  the  valve. 

2.  Turn  the  engine  forward  until  the  crank  end  dead  center  is 
reached  and  measure  the  lead  in  the  same  manner. 

3.  If  the  leads  are  equal  and  of  jbhe  required  amount,  no  further 
adjustment  is  needed. 

4.  If  the  leads  are  equal  but  not  of  the  required  amount,  move 
the  eccentric  forward  to  give  more  lead  or  backward  to  give  less 
lead,  as  required. 

5.  If  the  leads  are  unequal,  they  must  first  be  made  equal  by 
changing  the  length  of  the  valve  rod.     To  do  this,  take  half  of 


162  STEAM  ENGINES 

the  difference  between  the  leads  and  change  the  length  of  the 
valve  rod  by  this  amount,  lengthening  it  if  the  head  end  lead  is 
larger  or  shortening  it  if  the  crank  end  lead  is  larger.  This  will 
make  the  leads  equal.  Then  make  the  leads  the  required  amount 
by  the  method  indicated  in  (4)  above. 

After  the  valve  has  been  set  by  measurement,  as  above,  the 
engine  should  be  run  and  indicator  diagrams  taken.  The  indica- 
tor diagrams  will  show  whether  or  not  the  valve  is  set  properly, 
and  any  slight  readjustment  that  may  be  necessary  may  be  made 
after  an  inspection  of  the  diagrams. 

The  above  method  of  setting  valves  requires  considerable 
turning  of  the  engine  by  hand  which,  if  the  engine  is  large,  may 
be  inconvenient.  If  it  is  difficult  to  turn  the  engine  by  hand, 
the  following  method  of  setting  a  plain  slide  valve  may  be  used 
and  good  results  obtained.  This  method,  however,  is  suitable 
for  only  plain  slide  valve  engines  in  which  there  is  no  rocker  arm 
for  equalizing  the  cut-off  on  the  two  ends  of  the  cylinder. 

This  method  is  based  on  the  fact  that  a  slide  valve  without  an 
equalizing  rocker  arm  will  give  the  same  maximum  port  opening 
on  the  two  ends  of  the  cylinder  when  the  leads  are  equal.  The 
valve  is  therefore  first  set  to  give  the  same  maximum  port  open- 
ing on  the  two  ends  of  the  cylinder.  This  is  done  by  loosening 
the  eccentric  on  the  shaft  and  turning  it  around  until  it  gives 
maximum  port  opening  on  first  one  end  and  then  on  the  other. 
If  the  maximum  port  openings  are  not  equal  they  are  made  so 
by  changing  the  length  of  the  valve  stem  by  one-half  of  the 
difference  in  the  maximum  port  openings.  This  operation  gives 
the  valve  stem  its  proper  length.  The  engine  is  now  put  on  dead 
center  and  the  valve  given  the  proper  lead  by  turning  the  eccentric 
on  the  shaft.  This  adjusts  the  angle  of  advance  and  will 
give  equal  leads  at  the  two  ends  of  the  cylinder.  The  adjustment 
should  now  be  verified  by  indicator  diagrams  as  with  the  preceding 
method.  % 

Setting  Valves  for  Equal  Cut-off.— If  there  is  no  equalizing 
rocker  arm  the  steam  laps  and  leads  will  be  unequal  when  the 
valve  is  set  to  give  equal  cut-off  on  the  two  ends  of  the  cylinder 
and,  as  changing  the  length  of  the  valve  stem  is  equivalent  to 
making  the  laps  unequal,  most  of  the  adjustment  is  made  by  the 
valve  stem. 

The  engine  is  first  placed  exactly  on  its  head  end  dead  center, 
using  a  tram  for  this  purpose,  as  previously  described.  The 


VALVE  SETTING  163 

eccentric  is  then  loosened  and  turned  on  the  shaft  until  the  valve 
has  the  proper  lead  on  thp  head  end.  The  engine  is  then  moved 
forward  until  the  valve  coines  line  on  line  with  the  edge  of  the 
port,  which  is  its  position  at  cut-off.  Now  measure  the  displace- 
ment of  the  crosshead  from  the  beginning  of  its  stroke.  The 
engine  is  then  moved  forward  again  until  cut-off  occurs  on  the 
return  stroke,  and  the  displacement  of  the  crosshead  from  the 
crank  end  of  its  stroke  is  measured.  If  the  cut-off  on  the  head 
end  is  earlier  than  that  on  the  crank  end  the  valve  stem  is  too 
long,  but  if  the  cut-off  on  the  crank  end  is  earlier  than  that  on  the 
head  end  the  valve  stem  is  too  short.  In  either  case  the  length  of 
the  valve  stem  should  be  changed  by  an  amount  which  it  is  esti- 
mated will  correct  the  inequality  in  the  cut-offs.  Changing  the 
length  of  the  valve  stem  will,  of  course,  change  the  lead  on  the 
head  end;  therefore  the  engine  must  now  be  turned  to  the  head 
end  dead  center  and  the  lead  adjusted  to  its  original  amount  by 
moving  the  eccentric  around  on  the  shaft.  The  cut-offs  are  again 
measured  for  equality  and,  if  necessary,  the  length  of  the  valve 
stem  again  adjusted.  In  setting  valves  by  this  method,  a  valve 
diagram  will  often  prove  helpful  in  determining  the  amount  of 
adjustment  to  make  on  the  valve  stem. 

With  all  the  methods  of  setting  valves  described  above  it  is 
necessary  to  remove  the  cover  of  the  steam  chest.  It  is  con- 
venient sometimes  to  be  able  to  set  the  valves  without  removing 
the  steam  chest  cover  and  even  when  steam  is  on  the  engine.  This 
may  be  the  case  with  a  locomotive  when  out  on  the  road,  due  to 
the  eccentrics  slipping. 

In  order  to  be  able  to  set  valves  without  removing  the  steam 
chest  cover  it  is  first  necessary  to  set  the  valves  properly  while 
the  steam  chest  cover  is  off  and  then  make  reference  marks  on  the 
valve  stem  and  steam  chest. 

Preparatory  to  setting  the  valves  by  this  method  a  tram  with 
both  ends  pointed  and  bent  at  right  angles  should  be  made.  This 
tram  should  be  about  half  the  length  of  the  valve  stem.  The 
steam  chest  cover  is  then  removed  and  the  valve  set  with  equal 
leads  by  the  first  method  described  above.  After  the  valve  is 
properly  adjusted  the  engine  is  placed  on  the  head  end  dead 
center  as  shown  in  Fig.  98,  and  a  punch  mark  made  on  the  valve 
chest.  One  end  of  the  tram  is  placed  in  this  mark  and  a  mark 
made  on  the  valve  stem  with  the  other  end  of  the  tram.  The 
mark  on  the  valve  stem  is  then  made  permanent  by  a  punch 


164 


STEAM  ENGINES 


mark.  The  engine  is  then  turned  to  the  crank  end  dead  center, 
as  shown  in  Fig.  99,  and  with  the  tram  in  the  mark  on  the  valve 
chest,  another  mark  which  is  also  made  permanent  by  a  punch 
mark,  is  made  on  the  valve  stem.  The  valves  may  now  be  set 
at  any  time  without  removing  the  steam  chest  cover  by  placing 
the  engine  on  center  and  then  turning  the  eccentric  on  the  shaft 
until  the  tram  reaches  from  the  mark  on  the  valve  chest  to  the 
mark  on  the  valve  stem  which  corresponds  to  that  dead  center. 


FIG.  98. 

Types  of  Slide  Valves. — The  form  of  slide  valve  which  has  been 
illustrated  in  the  previous  discussion  is  known  as  the  plain  or  D 
slide  valve,  and  it  has  been  used  in  this  discussion  on  account  of 
its  simplicity.  This  form  of  valve  is  widely  used  on  the  cheaper 
grades  of  slide  valve  engines;  but  there  are  certain  objections  to 
its  use  on  better  grades  of  engines,  the  most  important  of  these 
objections  being  the  wire-drawing,  or  drop  in  pressure  produced 
by  it  during  admission,  and  its  friction. 


FIG.  99. 

Drop  in  pressure  during  admission  is  caused  by  friction  of 
the  steam  rushing  past  the  edges  of  the  valve  and  through  the 
narrow  port  opening  and  by  the  comparatively  slow  motion  of 
the  valve  while  it  is  opening  and  closing.  This  objection  is  not 
so  serious  as  might  be  first  thought  because  while  there  is  a  rather 
large  drop  in  pressure,  there  is  also  some  heat  produced  by  friction 
of  the  steam  which  tends  to  dry  the  admission  steam  if  it  is  wet 


VALVE  SETTING  165 

or  to  superheat  it  if  it  is  dry.  However,  on  the  whole,  the  drop 
in  pressure  during  admission  is  an  objection  and  some  engine 
builders  try  to  avoid  it  by  designing  their  valves  so  as  to  produce 
a  large  port  opening* with  a  small  movement  of  the  valve.  In  this 
way  the  valve  opens  quicker  and  wider  and  there  is  less  friction 
because  of  the  smaller  valve  travel. 

One  of  the  ways  in  which  these  results  have  been  accomplished 
is  by  the  use  of  double  ported  and  multiple  ported  valves. 

The  double  ported  valve  illustrated  in  Fig.  100  is  designed  to 
give  twice  as  large  port  opening  with  the  same  valve  movement  as 
would  be  obtained  with  the  ordinary  D  valve.  Steam  surrounds 
the  valve  and  also  fills  the  hollow  chambers  A  and  A  which  extend 


FIG.  100. 

entirely  through  the  valve  from  one  side  to  the  other.  The  bot- 
toms of  the  chambers  A  and  A  are  open  so  that  steam  may  flow 
from  them  into  the  ports.  The  ports  have  two  openings  so 
arranged  that  when  the  outside  edge  of  the  valve  uncovers  one 
opening  the  chamber  A  or  A  uncovers  the  other  opening  thus 
permitting  steam  to  enter  the  ports  at  two  points.  Exhaust 
occurs  past  the  inside  edges  of  the  valve  and  the  inside  edges  of 
the  chambers  A  and  A.  Passages  are  cored  over  the  tops  of 
the  chambers  A  and  A  so  that  steam  exhausted  past  the  in- 
side edge  of  the  valve  may  find  its  way  to  the  central  exhaust 
chamber. 

The  form  of  valve  shown  here  is  partly  balanced  by  means 
of  the  ring  H  which  slides  on  the  under  side  of  the  steam  chest 


166 


STEAM  ENGINES 


cover.  Any  leakage  of  high  pressure  steam  past  the  ring  finds 
its  way  into  the  exhaust  chamber  through  the  holes  M  and  M. 

This  type  of  valve  is  widely  used  on  marine  engines  and  it  is 
made  both  balanced  and  unbalanced.  The  smaller  sizes  of  valves 
are  often  unbalanced  but  the  larger  sizes  are  invariably  balanced. 

The  Trick  valve,  illustrated  in  Fig.  101  and  named  after  its 


FIG.  101. 

inventor,  is  a  form  of  double  admission  valve,  and  is  used  to  a 
considerable  extent  on  locomotives.  Live  steam  is  admitted 
past  the  outer  edge  of  this  valve  and  also  through  the  passage  A 
cast  in  the  body  of  the  valve.  When  the  outer  edge  of  the  valve 
uncovers  the  port  for  admission  the  opposite  end  of  the  passage  A 
is  also  uncovered  thus  giving  a  double  admission  of  steam,  the 


FIG.  102. 

same  as  with  a  double  ported  valve.  In  a  similar  way,  when  the 
outer  edge  of  the  valve  closes  the  port  for  cut-off  the  opposite 
end  of  the  passage  is  also  closed.  The  Trick  valve  gives  double 
admission  but  does  not  give  double  exhaust  since  the  passage  A 
is  not  open  to  exhaust  at  any  time.  In  this  respect  the  Trick 
valve  differs  from  a  double  ported  valve. 

The  Straight-line  valve  shown  in  Fig.  102  is  both  a  double 
admission  and  a  double  exhaust  valve.  This  result  is  secured  by 
means  of  two  ports  through  each  end  of  the  valve,  the  port  A 


VALVE  SETTING 


167 


being  for  admission,  and  the  port  B  for  exhaust.  Both  sides  of 
the  valve  are  exactly  alil^e  and  the  balance  plate  C  has  recesses 
cored  in  it  to  correspond  wjth  the  port  openings  in  the  cylinder 
valve-face.  This  manner  of  constructing  the  pressure  plate  per- 
mits almost  perfect  balancing  of  the  valve  since  both  sides  of  the 
valve  are  subjected  to  the  same  steam  pressure. 

The  passage  A  through  the  valve  gives  a  wide,  quick  opening 
and  thus  prevents  wire  drawing  during  admission.     The  passage 


FIG.  103. 

serves  the  same  object  with  the  exhaust,  preventing  wire  drawing 
at  release. 

The  Straight-line  valve  is  used  on  automatic  high  speed 
engines  which  are  controlled  by  a  shaft  governor.  The  shaft 
governor  is  connected  directly  to  the  valve  and  changes  the  lead 
and  angle  of  advance.  The  valve  is  well  adapted  for  this  purpose 
because  it  is  so  perfectly  balanced  that  but  little  power  is  required 
of  the  governor  in  changing  its  position. 

Multi-ported  valves  are  usually  of  the  gridiron  type,  consisting 
of  a  flat  plate  with  a  number  of  slots  in  it  which  slides  over  a  seat 
with  a  like  number  of  slots.  Valves  of  this  type  are  illustrated  in 
Fig.  103.  With  this  type  of  valve  a  large  port  opening  is  obtained 


168 


STEAM  ENGINES 


with  but  small  valve  travel.  For  example,  a  valve  of  this  type 
having  eight  slots  and  a  valve  travel  of  Y±  in.  would  have  a  port 
opening  of  two  inches,  while  an  ordinary  valve  would  have  a 
travel  of  two  inches  to  secure  the  same  amount  of  port  opening. 
The  smaller  travel  of  the  multi-ported  valve  reduces  its  friction 
and  the  amount  of  work  necessary  to  move  it,  and  at  the  same 
time,  makes  effective  lubrication  easier. 

Multi-ported  valves  are  usually  placed  across  the  cylinder,  as 
shown  in  Fig.  103,  instead  of  lengthwise  of  it,  and  there  are  usu- 
ally four  valves,  one  for  admission  to  each  end  of  the  cylinder  and 
one  for  exhaust  from  each  end.  This  method  of  placing  the 
valves  permits  shorter  ports  and  reduces  the  clearance. 


FIG.  104. 

Some  of  the  slide  valves  which  have  been  described  and  illus- 
trated are  balanced  or  partly  balanced.  Most  balanced  valves, 
except  piston  valves,  are  balanced  by  means  of  a  balance  plate 
over  which  the  valve  slides,  or  by  means  of  a  balance  ring  recessed 
into  the  back  of  the  valve  and  sliding  on  the  inner  surface  of  the 
steam  chest  cover  plate. 

The  excessive  unbalanced  pressure  on  the  common  D-valve 
which  causes  friction  and  cutting  is  mainly  due  to  the  large  ex- 
haust cavity  which  is  filled  with  low  pressure  steam  while  high 
pressure  steam  surrounds  the  outside  of  the  valve.  If  the  valve  is 
arranged  so  that  live  steam  may  be  admitted  to  the  cylinder  from 
the  inside  of  the  valve  while  the  outside  is  subjected  to  exhaust 
steam  the  unbalanced  pressure  is  greatly  reduced. 

The  Ball  telescopic  valve,  illustrated  in  Fig.  104,  is  designed  on 


VALVE  SETTING 


169 


this  principle.  This  valve  consists  of  two  parts,  one  of  which 
telescopes  into  the  other .»  'Each  part  consists  of  a  rectangular 
frame  which  slides  over  the^ports,  and  on  the  back  of  which  is  a 
short  hollow  cylinder.  The  cylindrical  parts  telescope  and  the 
inner  one  is  provided  with  packing  rings  to  prevent  leakage  of  the 
steam  from  the  inside  to  the  outside  of  the  valve. 

Steam  is  admitted  to  the  inside  of  the  valve  and  the  exhaust 
escapes  past  the  outside  edges.  Both  sides  of  the  valve  have 
working  edges  which  make  unnecessary  double  ports  and  large 
clearance  volume. 

Piston  valves  are  used  on  a  great  many  automatic  high  speed 


FIG.  105. 

engines.  This  type  of  engine  requires  a  balanced  valve  because 
the  governor  is  attached  directly  to  the  valve  and  governs  the 
speed  by  changing  the  position  of  the  valve.  If  a  large  amount  of 
power  is  required  to  move  the  valve  a  sensitive  governor  cannot 
be  used  with  it. 

Piston  valves  are  cylindrical  valves  moving  in  the  direction  of 
their  axis.  They  may  be  made  either  for  inside  or  for  outside 
admission.  The  steam  ports  consist  of  annular  spaces  surround- 
ing the  valve,  and  the  admission  and  exhaust  edges  extend  all 
around  the  circumference  of  the  valve,  hence  a  large  port  opening 
is  secured  with  a  small  diameter  of  valve. 

Packing  rings  to  prevent  leakage  of  steam  are  used  on  the 
larger  sizes  of  piston  valves  but  the  smaller  sizes  have  none.  The 


170  STEAM  ENGINES 

Ideal  piston  valve  illustrated  in  Fig.  105  shows  one  method  of 
using  packing  rings  on  piston  valves.  Only  one  end  of  the  valve 
is  shown  here,  the  other  end  being  exactly  like  this  one.  Two 
rings  A  are  used  on  each  end  of  the  valve  and  they  are  made  the 
exact  size  of  the  cylinder  bore.  These  are  not  " spring"  rings,  as 
they  would  cut  the  cylinder,  but  they  are  split  and  are  slightly 
adjustable  by  means  of  the  four  shoes  C.  Thus  the  rings  may  be 
adjusted  to  take  up  wear  and  keep  the  valve  steam  tight  at  all 
times. 


* 

* 


CHAPTER  XIII 
SHIFTING  ECCENTRIC  AND.  MEYER  VALVE 

Shifting  Eccentric. — In  the  plain  slide  valve  engine  the  eccen- 
tric is  fastened  to  the  shaft  by  means  of  a  set  screw  or  key,  hence, 
the  cut-off  occurs  at  a  fixed  point  in  the  stroke  and  cannot  be 
changed  unless  the  engine  is  stopped  and  the  eccentric  moved 
around  on  the  shaft.  This  would  change  the  angle  of  advance 
and  consequently  the  point  of  cut-off. 

An  inspection  of  the  valve  diagram,  Fig.  88,  will  show  that 
the  point  of  cut-off  with  a  plain  slide  valve  may  be  changed  by 
changing  either  the  angle  of  advance  or  trie  eccentricity.  Cut- 
off may  be  made  earlier  either  by  increasing  the  angle  of  advance 
or  decreasing  the  eccentricity  and  it  may  be  made  later  either 
by  decreasing  the  angle  of  advance  or  increasing  the  eccentricity. 

An  eccentric  constructed  in  such  manner  that  its  eccentricity 
and  angle  of  advance  may  be  changed  without  stopping  the  engine 
is  used  on  the  automatic  high  speed  type  of  engine.  This  type  of 
eccentric,  which  is  called  a  shifting  eccentric,  is  attached  to  the 
governor  in  such  manner  that  the  position  of  the  governor  controls 
the  eccentricity  and  angle  of  advance  and  by  this  means  regulates 
the  speed  of  the  engine  by  changing  the  point  of  cut-off  to  suit  the 
load  carried  by  the  engine.  A  device  of  this  kind  makes  it 
possible  to  use  the  simple  slide  valve  and,  at  the  same  time,  secure 
a  variable  cut-off  in  governing  the  speed  of  the  engine. 

The  principle  of  the  shifting  eccentric  is  illustrated  in  Fig. 
106.  The  eccentric  is  not  fastened  directly  to  the  shaft  but  to 
the  side  of  a  plate,  (7,  with  a  projecting  arm.  The  projecting 
arm  is  pivoted  at  A  to  a  point  on  one  of  the  spokes  of  the  fly  wheel. 
The  governing  mechanism  is  contained  in  the  flywheel  and  is 
attached  to  the  eccentric  on  the  side  opposite  the  projecting  arm, 
C,  hence,  the  eccentric  turns  with  the  flywheel.  The  shaft 
passes  through  a  slot  cut  in  the  eccentric,  the  width  of  the  slot 
being  a  little  greater  than  the  diameter  of  the  shaft.  The  slot 
is  curved  to  a  radius  OA  equal  to  the  distance  from  the  pivot  A 

171 


172 


STEAM  ENGINES 


to  the  center  of  the  shaft,  0,  so  that  the  eccentric  is  free  to  swing 
about  the  pivot,  A,  without  touching  the  shaft. 

The  governor,  which  is  attached  to  the  eccentric,  changes 
the  position  of  the  eccentric  with  respect  to  the  shaft  and  thus 
changes  the  eccentricity  and  angle  of  advance.  For  example, 
when  the  eccentric  and  shaft  occupy  the  positions  shown  by  the 
full  lines  in  Fig.  106  the  eccentricity,  which  is  the  distance  from 
the  center  of  the  shaft  to  the  center  of  the  eccentric,  is  OE  and  the 
angle  of  advance  is  the  angle  BOE.  If  the  eccentric  is  shifted 


[_L 


FIG.  106. 

until  the  shaft  occupies  the  position  indicated  by  the  dotted 
circle,  with  its  center  at  0',  the  eccentricity  will  be  O'E  and  the 
angle  of  advance  will  be  the  angle  BO'E.  It  will  be  observed 
that  in  shifting  from  the  first  to  the  second  position  the  eccen- 
tricity has  been  decreased  and  the  angle  of  advance  increased. 
Both  of  these  changes  have  the  effect  of  making  the  point  of 
cut-off  occur  earlier  in  the  stroke,  as  shown  by  the  valve  diagram 
in  Fig.  107.  In  this  valve  diagram  the  full  lines  represent  the 
first  position  of  the  eccentric,  with  cut-off  occurring  at  C,  and 
the  dotted  lines  represent  the  second  position  of  the  eccentric 


SHIFTING  ECCENTRIC  AND  MEYER  VALVE    173 

with  the  shorter  eccentricity  and  greater  angle  of  advance,  and 
the  cut-off  occurring  at  C'. 

The  manner  in  which  a  "swinging  eccentric  on  an  automatic 
high  speed  engine  is  operated  by  the  governor  is  illustrated  in 
Fig.  108.  The  eccentric  with  the  slot  and  shaft,  a,  is  shown  at  R. 
This  is  fastened  to  a  plate  T  which  is  pivoted  to  the  flywheel  at 
S  so  that  it  may  turn  about  the  point  S  and  thus  change  the 
eccentricity  and  angle  of  advance.  The  governor  consists  of  the 
weight  C,  the  link  H,  and  the  spring  E.  As  the  flywheel  turns, 


c' 


FIG.  107. 

the  weight,  C,  is  acted  upon  by  centrifugal  force  which  causes 
it  to  move  outward  from  the  center  of  the  flywheel.  The  out- 
ward movement  of  the  weight  is  resisted  by  the  spring  E  so  that 
for  any  particular  speed  the  weight  C  will  move  outward  until 
the  resisting  force  of  the  spring  just  balances  the  centrifugal 
force  acting  upon  the  weight.  The  weight,  C,  is  pivoted  to  the 
arm  of  the  flywheel  at  0,  and  the  link  H  is  pivoted  to  the  eccen- 
tric at  T.  When  the  weight,  C,  moves  outward  the  pivot  T  is 
moved  in  an  opposite  direction  and  the  center  of  the  eccentric 
is  moved  nearer  to  the  center  of  the  shaft.  This  decreases  the 
eccentricity  and  increases  the  angle  of  advance,  which  causes 

17 


174 


STEAM  ENGINES 


cut-off  to  occur  earlier  in  the  stroke  and  reduces  the  volume  of 
steam  supplied  to  the  cylinder  of  the  engine.  A  movement  of  the 
weight,  C,  towards  the  center  moves  the  eccentric  in  the  opposite 
direction,  lengthening  the  cut-off  and  admitting  a  greater  volume 
of  steam  to  the  cylinder. 

The  amount  of  centrifugal  force  acting  on  the  weight,  C, 
increases  with  an  increase  in  speed  and  decreases  with  a  decrease 
in  speed  of  the  engine.  If,  on  account  of  an  increase  in  load  on 
the  engine,  the  speed  should  be  decreased  the  centrifugal  force 
acting  on  C  would  become  smaller  and  the  spring  would  pull  C 
towards  the  center  of  the  shaft  and  move  the  center  of  the  eccen- 


FIG.  108. 

trie,  n,  away  from  the  center  of  the  shaft,  a;  thus  cut-off  would 
be  made  later  and  an  increased  amount  of  steam  would  be 
admitted  to  the  engine  to  make  it  go  faster.  So,  also,  if  on 
account  of  a  decrease  in  the  load  the  speed  of  the  engine  should  in- 
crease, the  centrifugal  force  would  become  greater,  and  C  would 
move  farther  away  from  the  center  of  the  shaft  thus  moving  the 
center  of  the  eccentric  towards  the  center  of  the  shaft,  making 
cut-off  earlier  and  reducing  the  amount  of  steam  admitted  to  the 
engine. 

The  spring,  E,  acts  as  a  controlling  force  upon  the  weight,  (7, 
and  regulates  its  position  for  any  given  speed,  therefore  the  stiff- 
ness of  the  spring  controls  the  speed  at  which  the  engine  will  run. 


SHIFTING  ECCENTRIC  AND  MEYER  VALVE    175 

The  stiffness  of  the  spring  may  be  changed  by  adjusting  the 
length  of  the  connection,  P,  between  the  spring  and  the  weight 
C,  which  is  provided  with  a  Hum  buckle  for  that  purpose. 

The  effects  of  different  ptffeitions  of  the  swinging  eccentric 
upon  the  distribution  of  steam  to  the  cylinder  may  be  studied 
from  a  valve  diagram  such  as  that  shown  in  Fig.  109.  The  swing- 
ing eccentric  gives  its  greatest  eccentricity  when  the  engine  is  at 
rest  or  when  the  load  is  the  greatest.  In  the  valve  diagram, 
Fig.  109,  the  line  OE  represents  this  eccentricity  and  the  valve 
circle  shows  the  cut-off  occurring  at  A  i.  The  arc  ED  is  drawn  with 
a  radius  equal  to  the  length  of  the  arm  on  which  the  eccentric 


FIG.  109. 

swings,  or  AO  in  Fig.  106.  As  the  cut-off  is  shortened  by  the  gov- 
ernor, the  eccentricity  becomes  smaller  and  for  any  position  of  the 
governor  the  eccentricity  will  be  the  distance  from  the  center  0 
of  Fig.  109  to  the  arc  ED.  Thus,  when  the  eccentric  has  been 
moved  one-half  of  its  total  swing,  its  eccentricity  will  be  OE' ', 
the  point  E'  being  located  halfway  between  E  and  D,  and  the 
corresponding  cut-off  will  occur  when  the  crank  is  in  the  position 
OA2.  A  valve  circle  drawn  on  OE'  will  give,  by  its  intersection 
with  the  lap  circle,  the  point  of  cut-off  Az  for  the  new  position 
of  the  eccentric,  since  the  lap  circle  is  the  same  for  all  positions 
of  the  eccentric.  In  a  similar  manner,  the  cut-off  for  any  position 
of  the  eccentric  may  be  found  by  drawing  a  line  from  0  to  a 


176  STEAM  ENGINES 

point  on  the  arc  ED  which  represents  the  position  of  the  eccentric, 
and  then  drawing  the  valve  circle  on  this  line. 

The  events  occurring  past  the  exhaust  side  of  the  valve  may 
be  located  by  extending  the  valve  circle  diameters  through  the 
center  0  and  drawing  valve  circles  on  them  of  the  same  size  as 
those  used  in  locating  the  point  of  cut-off.  In  this  way,  Fig. 
109  shows  that  the  point  of  compression  occurs  at  K\  when  the 
cut-off  occurs  at  Ai  and  that  the  point  of  compression  occurs  at 
Kz  when  the  cut-off  occurs  at  A2. 

It  will  be  observed  from  the  valve  diagram  that  the  lead  in- 
creases as  the  cut-off  is  shortened  or  the  load  becomes  lighter, 
and  that  the  amount  by  which  the  lead  changes  depends  upon 
the  length  of  the  projecting  arm  to  which  the  eccentric  is  fastened. 
When  the  cut-off  occurs  at  A  i  the  amount  of  lead  is  LM  and  when 
the  cut-off  occurs  at  A2  the  lead  is  LN.  The  automatic  high 
speed  engine  is  designed  to  run  at  practically  constant  speed  and 
to  change  the  point  of  cut-off  to  suit  the  load  carried.  The  lead 
has  a  cushioning  effect  upon  the  piston,  hence,  at  constant  speed 
the  lead  should  be  practically  constant  or  if  it  changes  at  all  it 
should  increase  when  the  point  of  cut-off  occurs  later  in  the  stroke, 
as  the  engine  is  then  carrying  its  heaviest  load.  The  fact  that 
the  swinging  eccentric  increases  the  lead  for  short  cut-offs  is 
sometimes  urged  as  an  objection  to  this  method  of  governing. 
The  lead  may  be  made  more  nearly  constant  by  pivoting  the 
eccentric  further  from  the  center  of  the  flywheel  since  this  gives 
a  flatter  arc  ED  on  the  valve  diagram,  but  there  will  always  be 
some  change  in  lead  with  this  kind  of  eccentric. 

A  study  of  the  valve  diagram  shows  also  that  the  period  of 
compression  is  increased  as  the  cut-off  becomes  shorter.  Com- 
pression cushions  the  piston,  hence  it  would  be  desirable  to  have 
a  constant  amount  of  compression  on  a  constant  speed  engine. 
However,  the  changes  in  compression  have  but  little  effect  upon 
the  smoothness  of  running  of  the, automatic  high  speed  engine 
on  account  of  the  large  clearance  volume  which  the  engines  have. 
The  large  clearance  volume,  made  necessary  by  the  high  speed 
of  the  engine,  flattens  the  compression  curve  on  the  indicator 
diagram,  showing  that  the  cushioning  effect  upon  the  piston  is 
applied  gradually,  hence  a  change  in  the  point  of  compression 
does  not  affect  the  cushioning  effect  as  much  as  it  would  on  an 
engine  with  smaller  clearance. 

The  action  of  the  flywheel  governor  and  swinging  eccentric 


SHIFTING  ECCENTRIC  AND  MEYER  VALVE    177 

may  be  seen  from  Fig.  110  which  shows  a  series  of  indicator 
diagrams  drawn  for  different  cut-offs.  These  diagrams  were 
drawn  when  the  engine  was  running  at  the  same  speed,  but  with 
different  loads.  The  change*  in  lead  are  particularly  noticeable 
on  these  diagrams,  the  one  with  the  latest  cut-off  showing  almost 
no  lead  and  the  others  showing  increasing  lead  as  the  cut-off 
is  shortened.  The  manner  in  which  the  "point  of  compression 
is  changed  with  the  cut-off  is  also  very  noticeable.  With  the 
latest  cut-off  the  point  of  compression  occurs  almost  at  the  end 
of  the  exhaust  stroke,  but,  with  no  load,  when  cut-off  is  earliest, 
the  point  of  compression  occurs  before  mid-stroke. 

The  valves  used  on  automatic  high  speed  engines  are  always 
balanced  valves,  balancing  being  secured  either  by  a  cover  plate 
on  the  back  of  the  valve  or  by  the  use  of  a  cylindrical  or  piston 
valve.  It  is  very  necessary,  in  these  engines,  that  the  power 


FIG.  110. 

required  to  move  the  valve  back  and  forth  be  reduced  to  a  mini- 
mum because  the  valve  is  moved  by  the  governor  and,  if  much 
power  is  required  to  move  it,  the  sensitiveness  of  the  governor 
will  be  reduced.  For  this  reason  the  valves  on  automatic  high 
speed  engines  are  balanced  and  friction  is  reduced  as  much  as 
possible  by  good  workmanship  in  making  the  valves. 

An  automatic  high  speed  engine  is  built  to  run  at  a  certain 
speed  and  the  governor  is  designed  and  adjusted  by  the  manufac- 
turer for  this  speed.  The  speed  may,  however,  be  changed  to  a 
certain  extent  either  by  changing  the  tension  of  the  springs  or  by 
decreasing  the  weights.  The  speed  will  be  increased  by  increas- 
ing the  tension  of  the  springs  or  by  decreasing  the  weights  as, 
in  either  case,  a  greater  speed  will  be  required  to  overcome  the 
tension  of  the  springs. 

Effects  Produced  by  Slide  Valve. — One  of  the  principal  objec- 
tions to  the  plain  slide  valve  engine  is  its  unfavorable  steam  distri- 


178 


STEAM  ENGINES 


bution.  All  of  the  events,  admission,  cut-off,  release,  and  com- 
pression are  controlled  by  a  single  valve  and  if  one  of  the  events 
is  changed  all  of  the  others  are  changed  in  proportion.  For 
example,  if  cut-off  is  made  to  occur  earlier  in  the  stroke  all  of  the 
other  events,  release,  compression,  and  admission  are  also  made 
to  occur  earlier. 

The  construction  of  a  plain  slide  valve  is  such  that  the  angle 
turned  through  by  the  flywheel  during  expansion  is  always  equal 
to  the  angle  turned  through  during  the  period  of  compression,  as 
an  inspection  of  the  valve  diagram  for  this  type  of  valve  will 
show.  Ordinarily  a  long  period  of  expansion  is  desirable  as  a 
greater  expansion  of  the  steam  is  then  secured  and  more  work 
obtained  from  the  steam,  but  this  can  be  obtained,  with  the  plain 


FIG.  111. 

slide  valve,  only  by  having  a  long  period  of  compression,  which 
reduces  the  amount  of  work  secured  from  the  steam.  With  these 
opposing  conditions  the  best  results  will  be  obtained  from  the 
engine  by  locating  the  point  of  cut-off  early  enough  so  there  will 
be  some  expansion  of  the  steam  but  not  enough  to  result  in 
excessive  compression.  For  this  reason  the  point  of  cut-off 
for  a  plain  slide  valve  engine  is  usually  made  to  occur  between 
one-half  and  seven-eighths  of  the  stroke.  If  it  occurs  earlier  than 
mid-stroke  the  compression  will  be  excessive,  and  if  it  occurs 
later  than  at  seven-eighths  of  the  stroke  there  will  be  but  little 
expansion  of  the  steam.  Under  the  latter  condition  the  pressure 
of  the  steam  at  release  will  be  almost  the  original  pressure  during 
admission  and  when  the  exhaust  valve  is  opened  this  pressure 
will  be  wasted. 


SHIFTING  ECCENTRIC  AND  MEYER  VALVE    179 

Meyer  Valve. — A  study  of  the  plain  slide  valve  shows  that  if 
the  point  of  cut-off  could  be  changed  without  changing  the  other 
events,  a  much  more  economical  steam  distribution  could  be 
obtained  by  cutting  off  th$>  steam  earlier  in  the  stroke  and 
allowing  it  to  expand  a  greater  number  of  times.  This  is  made 
plain  by  considering  Fig.  Ill  in  which  the  full  line  indicator 
diagram  is  from  a  plain  slide  valve  engine  and  the  dotted  line 
shows  the  shape  of  diagram  that  would  be  obtained  if  the  point 
of  cut-off  was  made  earlier  without  disturbing  any  of  the  other 
events.  These  diagrams  show  that  more  than  half  as  much  work 
is  done  with  the  early  cut-off  as  with  the  late  one,  but  that  less 


FIG.  112. 

than  half  as  much  steam  is  admitted  to  the  cylinder.  Therefore, 
more  work  is  obtained  per  pound  of  steam  with  the  early  cut-off 
than  with  the  late  one.  This  result  is  to  be  expected  since  very 
little  of  the  expansion  force  of  the  steam  is  used  when  a  late  cut- 
off is  employed. 

An  early  cut-off  by  a  slide  valve  may  be  obtained  by  means 
of  a  device  called  a  Meyer  valve.  The  Meyer  valve  consists  of  an 
auxiliary  valve  sliding  on  the  back  of  the  regular  or  main  slide 
valve,  as  illustrated  in  Fig.  112.  The  main  valve  is  like  a  plain 
slide  valve  except  that  instead  of  admitting  steam  past  the  outer 
edge,  the  valve  is  extended  and  has  two  ports,  P  and  PI  through  it 
and  the  admission  steam  flows  through  these  ports.  The  value 


180  STEAM  ENGINES 

is  constructed  in  this  way  in  order  to  provide  a  proper  surface 
on  which  the  auxiliary  or  riding  valve  may  slide. 

The  auxiliary  valve  consists  of  two  blocks,  Vi  and  Vz,  carried 
on  the  valve  rod  K  which  is  operated  by  a  separate  eccentric. 
The  auxiliary  valve  slides  on  the  back  of  the  main  valve  and,  at 
the  proper  instant,  covers  the  ports,  P  and  PI  through  the  main 
valve  thus  stopping  the  flow  of  steam  through  them  and  cutting 
off  the  steam  from  the  cylinder.  The  auxiliary  valve  will  close 
the  ports  P  and  PI  through  the  main  valve  and  cause  cut-off 
when  it  has  moved  with  respect  to  the  main  valve  a  distance 
equal  to  e,  in  Fig.  112,  which  is  called  the  clearance.  The  clear- 
ance may  be  adjusted  by  turning  the  handwheel,  W,  which  turns 
the  valve  rod.  The  valve  rod  is  provided  with  left-  and  right- 
hand  threads  so  that  turning  it  moves  the  blocks  Vi  and  Vz 
farther  apart  or  closer  together  and  thus  changes  the  clearance  e. 

The  main  valve  is  set  the  same  as  an  ordinary  slide  valve  but 
with  a  late  cut-off  in  order  to  secure  a  short  compression.  The 
eccentric  which  operates  the  auxiliary  valve  is  then  set,  usually  at 
180°  from  the  crank,  and  the  blocks  placed  on  the  valve  stem  in 
such  position  as  to  give  equal  cut-offs  on  the  two  ends.  This  is 
done  by  turning  the  blocks  around  separately  on  the  valve  stem 
until  the  clearance,  e}  is  adjusted  properly  to  equalize  the  cut-offs. 
The  point  of  cut-off  is  then  adjusted  for  both  ends  of  the  cylinder 
by  turning  the  valve  stem. 

The  auxiliary  valve  controls  only  the  cut-off  and  it  does  this 
by  closing  the  ports  through  the  main  valve  before  the  main 
valve  itself  cuts  off  in  the  regular  way.  The  main  valve  controls 
all  the  other  events,  release,  compression,  and  admission.  By 
setting  the  main  valve  to  give  a  short  compression  and  setting 
the  Meyer  valve  'to  give  an  early  cut-off,  an  indicator  diagram 
may  be  obtaine4  which  rerembles  very  closely  the  indicator 
diagram  obtained  from  a  Corliss  engine,  as  shown  by  the  dotted 
diagram  in  Fig.  111. 

The  action  of  the  Meyer  valve  may  best  be  examined  by  means 
of  a  valve  diagram  such  as  that  shown  in  Fig.  113.  In  this 
diagram  the  cricle  A  is  for  the  main  valve  and  the  circle  B  is  for 
the  auxiliary  valve.  The  diameter  of  A  is  located  so  that  the 
angle  JOF  is  equal  to  the  angle  between  the  crank  and  main 
eccentric  but  laid  out  in  a  direction  opposite  to  the  rotation 
as  indicated  by  the  arrow.  Likewise  the  diameter  of  the  auxiliary 
valve  circle,  OG,  is  laid  out  so  that  the  angle  JOG  is  equal  to  the 


SHIFTING  ECCENTRIC  AND  MEYER  VALVE    181 

whole  angle  between  the  crank  and  auxiliary  eccentric  but  in  a 
direction  opposite  to  the  rotation.  It  will  be  observed  that  while 
the  auxiliary  eccentric  is  usually  placed  directly  opposite  the 
crank,  in  this  case  it  is  placed  at  a  somewhat  greater  angle  than 
180°  in  order  to  make  the  diagram  more  general.  Both  the  main 
and  auxiliary  eccentrics  in  this  case  have  the  same  eccentricity 
as  they  usually  have  in  practice  but  this  is  not  at  all  necessary. 

Since  both  valves  are  moved  by  eccentrics,  their  relative 
motion  with  respect  to  each  other  may  be  represented  by  a  third 
valve  circle  C  which  is  drawn  with  its  diameter  OK  equal  and 
parallel  to  GF.  The  position  of  the  crank  at  cut-off  for  any 
amount  of  clearance  may  then  be  located  on  the  circle  C  by 


FIG.  113. 

drawing  a  line  OE  through  0  so  that  the  distance  OD  is  equal  to 
the  clearance.  Cut-off  will  then  occur  when  the  crank  is  in  the 
position  OE. 

The  circle  C  can  be  used  only  for  finding  the  point  of  cut-off 
on  one  end  of  the  cylinder.  In  order  to  find  the  cut-off  on  the 
other  end  the  line  KO  must  be  extended  and  another  circle  of 
the  same  size  as  C  drawn  on  it.  The  cut-off  for  the  other  end  of 
the  cylinder  may  then  be  located  on  it  in  the  same  manner  as  just 
described.  Since  admission,  release,  and  compression  are  con- 
trolled entirely  by  the  main  valve  they  must  be  located  by  means 
of  the  main  valve  circle.  Admission  is  located  by  drawing  the 
lap  circle  in  A  as  with  other  valve  diagrams  that  have  been  de- 
scribed before.  Compression  and  release  must  be  located  by 
extending  OF  and  drawing  on  it  another  valve  circle  of  the  same 
size  as  A.  By  drawing  the  exhaust  lap  arc  in  this  circle,  both 

18 


182  STEAM  ENGINES 

release  and  compression  may  be  located  the  same  as  on  other 
valve  diagrams. 

The  clearance  of  the  Meyer  valve  may  be  adjusted  to  cut  off 
the  supply  of  steam  at  any  point  between  the  beginning  of  the 
stroke  and  the  point  at  which  the  main  valve  cuts  off,  the  only 
limitations  being  that  the  valve  must  give  sufficient  port  opening 
at  the  beginning  of  the  stroke  and  must  not  re-open  before  the 
main  valve  closes,  a  matter  which  depends  upon  the  design  of  the 
valve. 

Engines  which  have  Meyer  valves  are  usually  governed  by 
means  of  a  throttling  governor  which  reduces  the  pressure  in  the 
steam  chest.  Attempts  have  been  made  to  operate  the  auxiliary 
valve  from  a  shifting  eccentric  connected  to  a  flywheel  governor 
as  is  done  in  the  automatic  high  speed  engine  but  this  method  is 
not  successful  with  the  Meyer  valve  because  it  gives  a  very 
unfavorable  steam  distribution.  If  the  swinging  eccentric 
lengthens  the  cut-off  on  one  end  of  the  cylinder  it  shortens  it  on 
the  other  because  such  a  governing  device  moves  the  auxiliary 
valve  as  a  whole,  whereas,  in  order  to  change  the  cut-off  equally 
on  both  ends  of  the  cylinder,  the  blocks  must  be  separated  or 
brought  closer  together.  This  can  only  be  done  by  .turning  the 
valve  rod,  and  a  swinging  eccentric  cannot  do  this. 

The  Meyer  valve  finds  its  most  successful  use  on  engines 
which  carry  a  fairly  uniform  load  such  as  on  those  running  air 
compressors  and  similar  machines.  For  this  class  of  service  the 
point  of  cut-off  may  be  adjusted  by  hand  to  the  most  favorable 
part  of  the  stroke,  which  will  be  as  early  as  the  load  will  permit, 
and  then  the  small  fluctuations  in  the  load  may  be  taken  care  of 
by  the  throttling  governor,  and  a  constant  speed  maintained. 
The  point  of  cut-off  may  be  changed  while  the  engine  is  running, 
since  the  handwheel  is  outside  the  valve  chest,  hence,  if  the  load 
changes  enough  to  require  a  different  point  of  cut-off,  it  may  be 
changed  to  suit  the  new  conditions.  The  handwheel  is  usually 
provided  with  a  pointer  and  scale  marked  in  the  fractions  of  the 
stroke  at  which  cut-off  occurs,  so  that  the  clearance  may  be 
adjusted  to  any  point  of  cut-off  by  simply  turning  the  handwheel 
until  the  pointer  is  at  the  fraction  at  which  it  is  desired  that  cut- 
off shall  occur. 


I 
I 


CHAPTER  XIV 
REVERSING  MECHANISMS 

Reversing  Gears. — Many  kinds  of  steam  engines  require  a 
form  of  valve  gear  by  means  of  which  the  direction  of  rotation 
may  be  readily  reversed.  Some  of  the  kinds  of  engines  which 
require  a  reversing  valve  gear  are  locomotives,  marine  engines, 
hoisting  and  winding  engines,  traction  engines,  and  rolling  mill 
engines. 

A  study  of  the  valve  diagram  shows  that  any  slide  valve 


CRANK 


FIG.  114. 

engine  may  be  reversed  by  simply  shifting  the  eccentric  around  on 
the  shaft.  For  an  outside  admission  valve  and  no  reversing 
rocker  arm  between  the  eccentric  and  the  valve,  the  eccentric 
is  set  ahead  of  the  crank  by  an  angle  somewhat  greater  than  90°. 
If  such  an  engine  is  to  run  in  a  clockwise  direction,  the  relative 
CRANK 


FIG.  115. 

positions  of  the  crank  and  eccentric  are  as  shown  in  Fig.  114. 
If  it  is  desired  to  have  this  engine  run  in  a  counterclockwise 
direction,  the  eccentric  must  be  loosened  and  turned  to  the 
position  shown  in  Fig.  115,  when  the  eccentric  will  be  ahead  of 
the  crank  but  in  a  counterclockwise  direction.  This  method 
of  reversing  an  engine  could  not  be  used  with  any  engine  which 
19  183 


184  STEAM  ENGINES 

required  a  quick  reversal  because  of  the  time  required  to  change 
the  position  of  the  eccentric  and  also  because  this  method  makes 
it  necessary  first  to  stop  the  engine. 

Many  forms  of  valve  gears  have  been  invented  which  will 
quickly  reverse  the  engine  without  having  to  change  the  position 
of  the  eccentric.  Some  of  these  mechanisms  make  use  of  two 
eccentrics,  one  for  each  direction  of  rotation;  some  of  them  use 
only  one  eccentric;  and  some  derive  their  motion  from  a  pin  on 
the  end  of  the  shaft  placed  with  its  center  a  short  distance  from 
the  center  of  the  shaft  so  as  to  give  the  same  motion  as  a  short 
crank.  This  arrangement  is  equivalent  to  a  single  eccentric. 
Some  of  the  more  common  forms  of  reversing  mechanism  are  the 
Stephenson  link  motion,  the  Walschaert  valve  gear,  and  the 
Woolf  reversing  gear. 


CRANK 


FIG.  116. 

Stephenson  Link  Motion. — The  Stephenson  link  motion  is  a 
reversing  mechanism  made  up  of  two  eccentrics,  two  eccentric 
rods  connected  by  a  link  and  a  single  valve  stem.  The  two  eccen- 
trics are  keyed  or  fastened  to  the  shaft  in  the  positions  shown  in 
Fig.  116.  The  eccentric  OE  is  in  the  proper  position  for  producing 
clockwise  rotation  and  the  eccentric  OE'  is  in  the  proper  position 
for  producing  rotation  in  a  counterclockwise  direction.  The 
mechanism  is  arranged  so  that  either  of  these  eccentrics  may  be 
made  to  operate  the  valve,  and  rotation  in  either  direction  pro- 
duced at  will. 

A  Stephenson  link  reversing  gear  is  illustrated  in  Fig.  117 
showing  the  relations  of  the  different  parts.  The  two  eccentrics 
are  shown  at  A  and  B  and  the  crank  at  C.  It  will  be  observed 
that  the  positions  of  the  two  eccentrics  in  relation  to  the  crank 


REVERSING  MECHANISMS 


185 


186  STEAM  ENGINES 

is  the  same  as  those  shown  in  Fig.  116.  The  eccentric  A  is  in  the 
proper  position  for  producing  rotation  in  a  clockwise  direction, 
and  B  is  in  the  proper  position  for  producing  rotation  in  a  counter- 
clockwise direction.  Each  eccentric  has  its  own  eccentric  rod 
connected  to  one  end  of  the  slotted  link  L.  The  link  L  is  sus- 
pended by  M  from  the  bell  crank  N  which  is  pivoted  at  R. 
The  bell  crank  is  operated  by  the  lever  P  so  that  by  moving  P 
the  link  L  may  be  raised  or  lowered.  The  end  of  the  valve  stem 
V  is  bolted  to  a  block  W  which  fits  in  the  slot  of  the  link  but  is 
free  to  move  in  it,  so  that,  as  the  link  is  raised  or  lowered,  the 
block  W  and  valve  stem  remain  stationary.  In  this  way  the 
valve  stem  may  be  brought  into  line  with  either  eccentric  rod  or 
it  may  be  given  any  intermediate  position  between  them. 

When  the  block  is  at  the  end  of  the  link,  and  the  valve  stem 
in  line  with  the  eccentric  rod  E}  as  shown  in  Fig.  117,  the  valve 
has  the  same  motion  as  if  connected  directly  to  the  eccentric 
A  and  the  engine  will  rotate  in  a  clockwise  direction.  In  this 
case  the  eccentric  B  has  no  effect  upon  the  motion  of  the  valve. 
Its  only  effect  is  to  swing  the  bottom  of  the  link  about  the  point 
a  as  a  pivot  in  the  same  manner  that  a  pendulum  swings. 

When  the  link  is  raised  so  that  the  valve  rod  is  in  line  with 
the  eccentric  rod  F,  all  of  the  motion  of  the  valve  comes  from 
the  eccentric  B  and  the  engine  therefore  runs  in  a  counter- 
clockwise direction.  In  this  case  the  only  effect  of  the  eccentric 
A  is  to  swing  the  upper  end  of  the  link  like  a  pendulum  about  the 
block  W  as  a  pivot  (the  block  now  being  at  the  lower  end  of  the 
link). 

When  the  link  is  raised  so  that  the  block  is  mid-way  between 
the  two  ends  of  the  link,  the  valve  is  acted  upon  equally  by  both 
eccentrics,  one  tending  to  produce  clockwise  rotation  and  the 
other  tending  to  produce  counterclockwise  rotation;  therefore, 
the  engine  will  not  run  in  either  direction. 

The  valve  mechanism  is  said  to  be  in  " mid-gear"  when  the 
block  is  in  the  middle  of  the  link  and  to  be  in  "full  gear"  when 
the  block  is  at  the  end  of  the  link.  There  are  two  "full  gear" 
positions,  one  called  "full  gear  foward"  and  the  other  "full  gear 
backward,"  to  indicate  the  position  of  the  link  which  will  cause 
the  engine  to  move  forward  or  backward. 

The  effects  upon  the  valve  motion  of  placing  the  block  in 
different  positions  in  the  link  and  also  the  effects  upon  the  steam 
distribution  to  the  cylinder  may  be  studied  by  means  of  a  valve 


REVERSING  MECHANISMS 


187 


diagram.  A  valve  diagram  for  a  Stephenson  link  motion  is 
shown  in  Fig.  118.  In  drawing  this  diagram  it  must  be  remem- 
bered that  when  the  valve  mechanism  is  in  "full  gear/'  the  motion 
of  the  valve  is  the  same  as  if  it  were  connected  to  one  of  the 
eccentrics  directly  and  the  other  eccentric  were  not  present; 
therefore  the  line  OE  represents  the  eccentricity  of  one  of  the 
eccentrics,  and  the  angle  COE  represents  the  angle  of  advance 
of  this  eccentric.  The  valve  circle  OAE  is  then  drawn  on  the 
line  OE  and  the  steam  lap  circle  AHJB  is  drawn  about  0  as  a 
center.  The  line  AF  then  gives  the  position  of  the  crank  at 


cut-off  when  the  valve  mechanism  is  in  full  gear  and  the  engine 
is  to  rotate  in  a  clockwise  direction. 

Both  eccentrics  of  a  Stephenson  link  motion  have  the  same 
eccentricity  and  the  same  angle  of  advance,  hence  the  location  of 
the  crank  for  full  gear  cut-off  when  the  engine  is  rotating  in  a 
counterclockwise  direction  is  found  on  the  valve  circle  OHE'B. 
The  diameter  of  this  valve  circle  is  the  same  as  OE  and  the  angle 
of  advance  DOE'  is  the  same  as  the  angle  COE.  Since  the  valve 
has  the  same  steam  lap  for  any  position  of  the  link,  cut-off  will 
occur  when  the  crank  is  in  the  position  OG. 


188  STEAM  ENGINES 

When  the  block  is  at  any  position  in  the  link  intermediate 
between  its  two  ends,  the  motion  of  the  valve  is  derived  from 
both  eccentrics  and  this  motion  might  be  produced  by  a  single 
imaginary  eccentric  having  a  smaller  eccentricity  and  a  greater 
angle  of  advance  than  either  of  the  actual  eccentrics. 

On  the  valve  diagram  the  center  of  the  eccentric  is  at  E  for 
one  full  gear  position  and  at  E'  for  the  other  full  gear  position. 
As  the  link  is  shifted,  the  center  of  the  imaginary  equivalent 
eccentric  moves  along  a  curved  path  EPE'  which  is  approximately 
the  arc  of  a  circle.  The  arc  EPE1  may  be  drawn  as  follows: 
Take  a  radius  equal  to 

7?--^ 
H  ~  2k 

in  which  s  is  the  distance  from  one  eccentric  center  to  the  other 
(the  distance  from  E  to  E'  on  the  valve  diagram),  I  is  the  length 
of  the  eccentric  rods,  and  k  is  the  length  across  the  link  measured 
from  the  center  of  the  block  in  one  full  gear  position  to  the  center 
of  the  block  in  the  other  full  gear  position.  With  a  center  on  the 
line  KL  and  a  radius  as  calculated  above  draw  an  arc  passing 
through  the  points  E  and  E',  cutting  KL  at  the  point  P.  A 
valve  circle  drawn  with  OP  as  a  diameter  gives  the  location  of 
the  crank  at  cut-off  OM  for  the  mid-gear  position  of  the  valve 
mechanism. 

For  any  position  of  the  link  intermediate  between  full  gear 
and  mid-gear  the  point  of  cut-off  may  be  located  by  drawing  a 
valve  circle  for  that  position  of  the  link.  This  is  done  by  dividing 
the  arc  EPE'  in  the  same  proportion  that  the  link  is  divided  by 
the  position  of  the  block.  For  a  position  of  the  block  half  way 
between  the  mid-gear  and  the  full  gear  positions,  the  valve 
circle  is  drawn  on  OR  as  a  diameter,  R  being  half  way  between 
E  and  P,  and  it  is  seen  that  cut-off  occurs  when  the  crank  is  in 
the  position  ON. 

It  will  be  observed  from  Fig.  118  that  the  latest  cut-off  is 
obtained  with  the  link  in  its  full  gear  position  and  that  as  the 
link  is  brought  towards  the  mid-gear  position  the  cut-off  becomes 
earlier,  being  at  ON  for  one-quarter  gear,  and  at  OM  for  mid- 
gear.  The  Stephenson  link  motion  may  therefore  be  used  to  a 
certain  extent  as  a  governor  since  the  point  of  cut-off  may  be 
regulated  by  it.  On  locomotives,  where  this  form  of  valve  gear 
is  commonly  used,  the  speed  is  regulated  by  both  the  throttle 
valve  and  the  valve  gear.  In  starting  a  locomotive  pulling  a 


REVERSING  MECHANISMS  189 

load  the  link  motion  is  put  in  full  gear  where  the  latest  cut-off 
is  obtained.  The  full  steam  pressure  then  acts  upon  the  piston 
for  nearly  the  entire  stroke.,.  As  the  locomotive  comes  up  to 
speed,  the  link  motion  is  gradually  notched  up  towards  mid-gear 
which  shortens  the  cut-off  and  allows  more  of  the  expansive  force 
of  the  steam  to  be  used. 

The  valve  diagram  shown  in  Fig.  118  is  not  complete,  only 
enough  of  it  being  shown  to  illustrate  the  method  of  drawing  it. 
The  remainder  of  the  diagram  is  omitted  in  order  not  to  complicate 
the  figure.  For  the  full  gear  position  the  line  OE  would  be 
extended  to  the  other  side  of  the  center  E  and  another  valve 
circle  drawn  on  it  with  the  exhaust  lap  to  locate  the  positions 
of  the  crank  at  release  and  compression,  in  the  same  manner  as 
for  an  ordinary  slide  valve  diagram.  These  two  valve  circles 
will  give  all  of  the  full  gear  events  for  one  end  of  the  cylinder. 
The  events  for  the  other  end  of  the  cylinder  would  be  found  by 
drawing  an  exhaust  lap  circle  in  the  valve  circle  EO  and  a  steam 
lap  circle  in  the  other  valve  circle.  The  same  process  would  be 
followed  in  drawing  a  complete  diagram  for  any  other  position 
of  the  link. 

The  link  motion  illustrated  in  Fig.  117  is  known  as  the  open 
rod  construction  and  the  valve  diagram  shown  in  Fig.  118  is 
for  this  form  of  link  motion.  There  is  another  form  of  link 
motion  called  the  crossed  rod  construction  in  which  the  eccentric 
rod  E}  Fig.  1 17,  is  attached  to  the  end/  of  the  link,  and  the  eccen- 
tric rod  F  is  attached  to  the  end  d  of  the  link.  With  either  the 
open  or  crossed  rod  construction  the  eccentric  rods  are  alter- 
nately open  and  crossed  every  half  revolution,  but  these  two 
constructions  may  be  distinguished  by  means  of  the  following 
rule:  For  an  outside  admission  valve  and  no  reversing  rocker 
arm  the  link  motion  is  open  rod  construction  if  the  rods  are  open 
when  the  crank  is  in  the  dead  center  position  on  the  side  of  the 
shaft  away  from  the  valve.  Under  the  same  conditions  the 
valve  gear  is  of  the  crossed  rod  construction  if  the  rods  are 
crossed  when  the  crank  is  in  dead  center  position  on  the  side 
away  from  the  valve  gear. 

The  motion  of  the  valve  for  crossed  rods  is  very  different  from 
its  motion  with  open  rods.  For  this  reason,  in  dismounting  a 
Stephenson  link  motion  it  is  very  necessary  to  know  whether  the 
link  motion  is  open  or  crossed  rod  construction  and,  in  reassem- 
bling, not  to  change  from  one  construction  to  the  other.  A  center- 


190 


STEAM  ENGINES 


line  diagram  of  a  crossed  rod  link  motion  is  illustrated  in  Fig. 
119. 

The  valve  diagram  for  the   crossed  rod  construction  is  the 
same  as  that  for  the  open  rod  construction  except  that  the  arc 


FIG.   119. 


ERPE'  Fig.  118  is  drawn  so  as  to  curve  in  the  opposite  direction. 
This  is  done  by  placing  the  center  of  the  arc  on  the  opposite  side 
of  the  center  of  the  diagram.  The  method  of  finding  the  length 
of  radius  used  for  drawing  the  arc  with  both  the  open  and  crossed 


rod  constructions  is  the  same.     The  valve  diagram  for  crossed 
rods  is  illustrated  by  Fig.  120. 

It  will  be  observed  from  Figs.  118  and  120  that  with  the  open 
rod  construction  the  lead  increases  as  the  link  is  moved  from 


REVERSING  MECHANISMS  191 

full  gear  to  mid-gear  while  with  the  crossed  rod  construction  the 
lead  decreases  as  the  link  v is"  moved  from  full  gear  to  mid-gear. 
The  fact  that  the  lead  chants  for  different  positions  of  the  link 
is  sometimes  urged  as  a  disadvantage  of  the  Stephenson  link 
motion,  and  some  of  the  later  types  of  reversing  mechanism  are 
designed  to  give  constant  lead.  For  use  on  locomotives,  if  the 
lead  must  change,  it  is  desirable  to  have  it  increase  from  full  gear 
to  mid-gear  in  order  to  give  more  cushioning  effect  as  the  engine 
speeds  up. 

In  setting  the  valve  of  a  Stephenson  link  motion  the  link  is 
placed  in  full  gear  and  the  same  method  is  then  used  as  in  setting 
the  ordinary  slide  valve.  The  precaution  must  be  taken  however 
to  see  that  the  eccentric  rods  are  of  the  same  length;  otherwise 
the  steam  distribution  will  not  be  the  same  for  forward  running 
as  for  backward  running. 

Walschaert  Valve  Gear. — While  the  Stephenson  link  motion 
is  used  on  most  American  locomotives  at  the  present  time,  it  is 


FIG.   121. 

being  displaced  on  the  later  types  of  locomotives  by  a  form  of 
reversing  mechanism  called  the  Walschaert  valve  gear. 

The  Stephenson  link  motion  and  other  types  of  link  motions 
have  two  eccentrics,  one  for  forward  motion  and  the  other  for 
backward  motion.  The  Walschaert  reversing  mechanism,  and 
others  of  the  same  type  have  but  one  eccentric  or,  in  some  cases, 
only  a  pin  on  the  end  of  the  crank  shaft  which  acts  as  a  crank 
of  small  throw.  Reversing  mechanisms  having  but  one  eccentric 
are  called  radial  valve  gears. 

With  the  Walschaert  valve  gear,  the  valve  takes  part  of  its 
motion  from  the  crank  shaft  and  part  from  the  crosshead,  these 
two  motions  being  combined  into  the  actual  motion  of  the  valve. 
The  part  of  the  motion  that  comes  from  the  crank  shaft  is  pro- 
duced by  connecting  the  valve  to  a  pin  set  at  90°  to  the  crank. 
A  valve  connected  directly  to  a  pin  placed  at  90°  to  the  main 
crank  is  shown  in  Fig.  121  in  which  OC  is  the  crank  and  E  is  the 


192 


STEAM  ENGINES 


pin  which  operates  the  valve.  The  small  crank  OE  may  be 
spoken  of  as  an  eccentric  since  it  serves  all  the  purposes  of  an 
eccentric. 

An  engine  fitted  with  a  mechanism  like  that  shown  in  Fig. 
121  will  not  run  if  the  valve  has  steam  lap,  because,  when  the 
piston  is  at  the  end  of  its  stroke  the  valve  is  in  its  mid-position 
arid  covers  the  steam  ports  a  distance  equal  to  the  steam  lap. 
In  order  to  allow  the  engine  to  run  the  valve  must  be  displaced 
a  distance  equal  to  the  steam  lap  when  the  crank  is  in  the  position 
shown  in  Fig.  121.  The  valve  will  then  be  on  the  point  of  open- 
ing when  the  piston  reaches  the  end  of  its  stroke.  For  proper 
operation  the  valve  should  also  be  given  some  lead;  that  is, 
when  the  piston  is  at  the  end  of  its  stroke,  as  shown  in  Fig.  121, 
the  valve  should  be  displaced  from  its  mid-position  a  distance 
equal  to  the  steam  lap  plus  the  lead. 


—  J»-  LAP*  LEAD 


Fio.  122. 

The  method  of  securing  lead  with  the  Walschaert  valve  gear 
is  shown  in  Fig.  122.  The  valve  stem  is  not  connected  directly 
to  the  eccentric  rod  but  to  a  combining  lever  which  is  connected 
to  the  crosshead.  The  eccentric  rod  is  also  attached  to  the  com- 
bining lever,  so  that  its  swing  is  due  both  to  the  motion  of  the 
crosshead  and  to  the  eccentric  crank  OE.  The  crosshead  carries 
a  drop  bar  BD  fastened  rigidly  to  it  and  the  combining  lever  is 
connected  to  it  by  a  union  link.  The  use  of  the  union  link  is 
made  necessary  by  the  fact  that  the  crosshead  travels  in  a 
straight  line  while  the  lower  end  of  the  combining  lever  swings 
in  the  arc  of  a  circle.  The  rise  and  fall  of  the  lower  end  of  the 
combining  lever  is  therefore  taken  up  by  a  slight  swing  of  the 
union  link  about  the  point  D,  thus  permitting  the  valve  stem  to 
travel  in  a  straight  line  through  its  stuffing  box. 

The  combining  lever  is  supported  at  the  point  where  the 
eccentric  rod  is  attached  to  it  and  this  point  forms  the  fulcrum 


REVERSING  MECHANISMS 


193 


of  the  lever.  The  motion  of  the  crosshead  then  displaces  the 
valve  independently  of  the --motion  produced  by  the  eccentric 
crank.  The  combining  lever  is  in  a  vertical  position  when  the 
crosshead  is  at  the  middle  point  of  its  travel;  therefore,  when  the 
piston  is  at  either  end  of  its  travel  the  valve  is  displaced  from  its 
mid- position  a  distance  depending  upon  the  distance  from  the  end 
of  the  combining  lever  to  the  point  at  which  the  eccentric  rod  is 
attached.  This  distance  is  made  great  enough  to  displace  the 
valve  a  distance  equal  to  the  steam  lap  plus  the  lead,  as  shown  in 
Fig.  122.  When  the  crank  is  on  the  back  dead  center,  as  shown 
in  Fig.  122,  the  combining  lever  displaces  the  valve  to  the  right 
of  its  mid-position  and  when  the  crank  is  on  the  other  dead 


FIG.  123. 

center,  the  valve  is  displaced  to  the  left  of  its  mid-position,  thus 
giving  lead  at  both. ends  of  the  piston  stroke. 

By  erecting  the  mechanism  so  that  the  combining  lever  stands 
in  a  vertical  position  when  the  piston  is  at  the  middle  point  of  its 
stroke,  the  displacement  of  the  valve  is  made  equal  for  each  dead 
center  position  of  the  crank.  If  the  valve  has  the  same  steam 
lap  on  both  sides,  the  leads  will  then  be  equal.  Moreover  the 
lead  will  be  constant  since  it  depends  only  upon  the  proportions 
of  the  combining  lever,  which  are  fixed. 

While  the  valve  gear  illustrated  in  Fig.  122  gives  the  valve 
the  proper  lead  and  will  run  the  engine  correctly,  the  direction  of 
rotation  of  the  engine  cannot  be  reversed  nor  can  the  point  of 
cut-off  be  changed.  Both  of  these  objects  are  accomplished  in 
the  Walschaert  valve  gear  in  the  manner  illustrated  in  Fig.  123. 
Instead  of  a  single  eccentric  rod  extending  from  the  eccentric 


194 


STEAM  ENGINES 


crank  to  the  combining  lever,  the 


eccentric  rod  is  divided  into 
two  parts  and  a  link 
interposed  between 
them.  The  link  is 
supported  at  its  center, 
about  which  it  oscillates. 
The  part  of  the  eccentric 
rod  which  extends  from 
the  link  to  the  combin- 
ing lever  (called  a  radius 
bar)  has  a  block  on  its 
end  which  may  be  moved 
up  or  down  in  the  link. 
Moving  the  block  from 
one  end  to  the  other  of 
the  link  reverses  the 
movement  of  the  valve 
and  therefore  reverses 
the  direction  of  rotation 
^.  of  the  engine.  The 
S  point  of  cut-off  is 
g  changed  as  the  block  is 
moved  from  the  center 
of  the  link  towards  the 
end.  The  latest  cut-off 
occurs  when  the  block  is 
at  the  end  of  the  link  for 
then  .the  valve  travel  is 
greatest  and  the  valve 
uncovers  the  steam 
ports  the  greatest 
amount.  As  the  block 
is  moved  towards  the 
center  of  the  link,  the 
valve  uncovers  the  ports 
a  smaller  amount  and 
the  cut-off  is  shortened. 
By  curving  the  link 
to  a  radius  equal  to 
the  length  of  the  radius 
bar  the  lead  is  kept  con- 


REVERSING  MECHANISMS  195 

stant  for  all  positions  of  the  block  in  the  link.  This  is  one  of 
the  points  of  difference  between  the  Stephenson  link  motion  and 
the  Walschaert  valve  gearv  The  former  varies  the  lead  for 
different  points  of  cut-off  and  the  latter  keeps  the  lead  constant. 
A  valve  gear  may  be  tested  for  constant  lead  by  shifting  from 
one  full  gear  position  to  the  opposite  and  watching  the  valve  or 
valve  rod.  With  constant  lead  the  valve  will  not  move  as  the 
mechanism  is  shifted  from  one  full  gear  position  to  the  other. 

An  example  of  the  Walschaert  valve  gear  as  applied  to  a 
locomotive  is  illustrated  in  Fig.  124.  It  will  be  observed  that  the 
eccentric  rod  is  not  connected  directly  to  the  lower  end  of  the 
link  but  rather  to  an  arm  which  projects  from  the  lower  end  of  the 
link  and  which  inclines  backward.  This  is  made  necessary  by 
the  fact  that  the  eccentric  rod  is  not  horizontal  but  inclines  at  a 
considerable  angle.  This  angularity  of  the  eccentric  rod  distorts 
the  motion  of  the  valve.  The  projecting  arm  on  the  link  is  added 
to  bring  the  eccentric  rod  as  near  the  horizontal  as  possible;  and 
the  arm  is  shaped  so  as  to  correct  the  distortion  in  the  valve 
motion  produced  by  the  angularity  of  the  eccentric  rod. 

The  Walschaert  valve  gear  is  not  a  new  device  since,  it  was 
invented  in  the  year  1848  and  has  been  in  more  or  less  extensive 
use  on  European  locomotives  since  then.  It  is  only  within  recent 
years,  however,  that  it  has  been  used  to  any  extent  in  the  United 
States.  Its  many  advantages  over  the  Stephenson  link  motion  is 
now  causing  its  rapid  adoption  on  locomotives  in  this  country. 

The  Walschaert  gear  is  much  lighter  in  weight  than  the 
Stephenson  link  motion.  The  Stephenson  link  motion  has  two 
heavy  eccentrics  with  their  straps  and  eccentric  rods  while  the 
Walschaert  gear  is  made  up  entirely  of  comparatively  light  rods. 
On  some  of  the  largest  locomotives  the  Stephenson  link  motion 
weighs  as  much  as  two  tons.  Two  reversals  of  this  large  weight 
in  each  revolution  with  a  fast  running  locomotive  throws  a  great 
strain  upon  the  engine. 

It  is  desirable  that  any  valve  gear  keep  its  adjustment.  In 
this  respect  the  Walschaert  is  superior  to  the  Stephenson  gear. 
The  joints  in  its  moving  parts  are  all  pin  joints  with  hardened 
bushings  which  reduce  wear  to  a  minimum  and  which  may  be 
easily  renewed  should  wear  occur.  In  the  Stephenson  link  motion 
there  is  a  certain  amount  of  sliding  between  the  block  and  the 
link  called  the  "slip."  The  two  eccentrics  are  also  subject  to 
considerable  wear.  Both  of  these  parts  are  under  a  locomotive 


196  STEAM  ENGINES 

and  exposed  to  the  dust  and  grit  from  the  road  bed,  which  causes 
them  to  wear  and  cut  very  fast  so  that  the  Stephenson  motion 
soon  loses  its  adjustment. 

The  Stephenson  link  motion  is  usually  placed  between  the 
drive  wheels  of  a  locomotive.  It  is  therefore  not  so  accessible 
as  the  Walschaert  gear  which  is  placed  outside  the  drive  wheels. 
The  more  accessible  location  of  the  valve  gear  insures  its  receiving 
better  attention  from  the  engineer,  especially  in  the  matter  of 
lubrication.  It  might  be  thought  that  the  more  exposed  location 
of  the  Walschaert  gear  renders  it  more  liable  to  damage,  but 
experience  shows  that  it  is  no  more  liable  to  injury  than  the 
Stephenson  link  motion. 

The  amount  of  space  occupied  by  the  Stephenson  link  motion 
under  a  locomotive  has,  in  some  cases,  interfered  seriously 
with  the  proper  bracing  of  the  frame.  The  location  of  the 


FIG.  125. 

Walschaert  gear  outside  the  drive  wheels  leaves  the  space  under 
the  locomotive  free  for  proper  bracing,  and  it  does  not  interfere 
with  the  design  of  any  other  part  of  the  locomotive. 

The  varying  lead  of  the  Stephenson  link  motion  is  urged  as 
an  objection  by  some  engineers  who  prefer  the  constant  lead  of 
the  Walschaert  gear. 

Woolf  Reversing  Gear. — This  is  a  simple  form  of  single  eccen- 
tric valve  gear  used  on  a  great  many  traction  and  similar  engines. 
This  form  of  valve  mechanism  is  illustrated  in  Fig.  125.  The 
eccentric  is  set  180°  from  the  crank.  The  eccentric  strap  carries 
a  projecting  arm  which  extends  upward  a  little  in  front  of  and 
inclined  to  a  vertical  line  through  the  center  of  the  shaft.  The 
outer  end  of  this  arm  carries  a  block  which  slides  in  a  straight 
but  inclined  guide.  The  eccentric  rod  is  attached  to  the  eccentric 
arm  a  short  distance  below  the  block,  and  extends  in  a  direction 
almost  at  right  angles  to  the  eccentric  arm.  The  other  end  of 


REVERSING  MECHANISMS 


197 


the  eccentric  rod  is  connected  to  a  rocker  arm  to  which  the  valve 
stem  is  attached.  ^f 

The  action  of  the  valve  ggar  may  be  seen  more  clearly  from 
the  center-line  diagram  shown  in  Fig.  126.  The  block  Q  on  the 
upper  end  of  the  eccentric  arm  moves  in  a  straight  inclined  line  in 
the  guides  while  the  lower  end  moves  in  a  circle.  This  makes 
the  point  P  at  which  the  eccentric  rod  is  attached  move  in  an 
ellipse.  The  engine  is  reversed  by  throwing  the  guide  over  to 
the  dotted  position.  The  point  P  then  follows  the  path  of  the 
dotted  ellipse.  The  point  of  cut-off  may  be  varied  by  changing 


FIG.  126. 

the  inclination  of  the  guide,  and  the  reverse  lever  is  supplied 
with  a  notched  sector  for  this  purpose. 

The  Woolf  reversing  gear  gives  a. constant  lead  for  all  degrees 
of  cut-off  but  the  leads  for  the  two  ends  of  the  cylinder  are  not 
necessarily  equal  The  steam  distribution  is  not  the  same  for 
both  directions  but  the  mechanism  is  usually  designed  to  give 
equal  cut-off  for  both  ends  of  the  cylinder.  Placing  the  center 
of  the  guides  a  little  forward  of  the  center  of  the  shaft  makes  the 
steam  distribution  more  nearly  alike  for  both  directions'  of 
rotation. 

The  simplicity  of  this  reversing  gear,  together  with  the  fact 
that  it  gives  a  fairly  good  steam  distribution,  has  made  it  very 
popular  for  traction  and  thrashing  engines  where  the  engineer  is 
usually  not  an  expert. 


CHAPTER  XV 
CORLISS  VALVE  GEARS 

Advantages  of  the  Corliss  Valve. — The  Corliss  valve  mechan- 
ism has  been  described  in  a  previous  chapter  and  the  two  admis- 
sion and  two  exhaust  valves  shown.  The  use  of  four  valves  on 
a  steam  engine  has  three  distinct  advantages.  They  may  be 
located  close  to  the  cylinder  bore  and  long  ports  thus  avoided. 
This  reduces  the  clearance  volume  and  the  surfaces  upon  which 
steam  may  be  condensed.  When  a  single  valve  is  used  to  control 
both  admission  and  exhaust,  it  is  first  heated  by  the  live  steam 
passing  it  and  then  cooled  by  the  exhaust  steam  passing  it. 
This  promotes  condensation  of  steam  upon  the  surfaces  of  the 
cooled  valve.  The  use  of  separate  valves  for  admission  and 
exhaust  reduces  this  condensation.  The  use  of  four  valves  has 
the  further  advantage  that  each  one  may  be  adjusted  separately, 
which  simplifies  the  operation  of  setting  them. ' 

Besides  the  advantages  which  arise  from  the  separation  of  the 
admission  and  exhaust  functions  the  Corliss  mechanism  gives 
an  almost  ideal  motion  to  the  valves.  The  valves  move  quickly 
while  they  are  opening  and  closing  thus  giving  sharp  and  well- 
defined  events  of  admission,  cut-off,  release,  and  compression 
and  also  avoiding  wire-drawing  during  admission.  The  valves 
have  but  little  motion  after  they  are  closed,  when  motion  is  not 
needed,  and  the  motion  at  this  time  is  slow.  This  reduces  the 
friction  of  the  valve  and  makes  good  lubrication  possible. 

The  slow  motion  of  the  valves  after  they  are  closed  is  due  to 
the  angle  at  which  the  valve  rods  are  connected  to  the  wrist  plate. 
This  angle  is  such  that  the  valves  are  given  a  rapid  motion 
when  the  wrist  plate  is  near  its  central  position  and  a  slower 
motion  as  the  wrist  plate  approaches  either  extreme  position. 
Fig.  127  will  make  this  clear.  The  heavy  line  CC'  shows  the 
position  of  the  steam  valve  rod  when  the  wrist  plate  is  in  its 
extreme  position  towards  the  left  and  the  steam  hook  picks 
up  the  valve.  As  the  wrist  plate  moves  to  the  right  through  the 
angle  from  C  to  B  the  steam  arm  moves  through  the  angle 

198 


CORLISS  VALVE  GEARS 


199 


between  C"  and  Br.  The  valve  remains  closed  during  this  period. 
A  further  movement  of  the  wrist  plate  through  the  angle  between 
B  and  A  moves  the  steam  arm -through  the  angle  between  Br  and 
A'  and  the  valve  is  opened  during  this  period.  It  will  be  observed 
that  although  the  wrist  plate  moves  through  a  smaller  angle 
while  the  valve  is  open  than  it  does  while  the  valve  is  closed  that 
the  steam  arm  (and  also  the  valve)  moves  through  a  larger  angle 
while  the  valve  is  open  than  while  it  is  closed.  Hence  the  valve 


FIG.  127. 

has  a  slow  motion  while  it  is  closed  and  a  fast  motion  while  it  is 
open.  This  effect  is  even  more  pronounced  since  the  wrist 
plate  moves  slower  when  near  its  extreme  position  (when  the 
valve  is  closed)  than  it  does  when  near  its  central  position. 

The  above  remarks  apply  only  to  the  opening  of  the  steam 

valve,  but  the  same  effect  is  produced  by  the  dashpot  in  closing 

the  valve.     As  soon  as  the  steam  valve  is  released  the  dashpot 

moves  rapidly  and  closes  the  valve.     When  the  dashpot  piston 

20 


200 


STEAM  ENGINES 


is  near  the  end  of  its  downward  stroke,  it  moves  more  slowly  in 
seating  gradually  and  the  valve,  which  is  closed  by  this  time, 
also  moves  slowly. 

The  exhaust  valves  are  connected  to  the  wrist  plate  at  all 
times  but  they  also  have  a  slower  motion  when  closed  than  when 
opening.  It  will  be  observed  that  the  wrist  plate  moves  through 
the  small  angle  between  F  and  E  while  the  exhaust  valve  arm 
moves  through  the  large  angle  between  Ff  and  Ef.  The  exhaust 
valve  is  open  during  this  period.  While  the  wrist  plate  moves 
through  the  large  angle  between  E  and  D  the  exhaust  valve  arm 
moves  through  the  small  angle  between  E'  and  D',  the  valve  being 
closed  during  this  period.  Moreover,  the  motion  of  the  wrist 

cc" 


FIG.   128. 

plate  is  slower  while  moving  from  E  and  D  than  while  moving 
from  F  and  E.  Each  valve  rod  is  usually  connected  to  the  wrist 
plate  in  such  position  that  a  line  through  its  axis,  if  extended, 
would  pass  through  the  center  of  the  wrist  plate  when  it  is  in  its 
extreme  position. 

Single  Eccentric  Valve  Gear. — The  valve  gear  of  some  Corliss 
engines  is  operated  by  a  single  eccentric  while  the  valve  gear  of 
others  is  operated  by  two  eccentrics.  With  a  single  eccentric 
the  cut-off  may  occur  anywhere  between  the  beginning  and 
35  per  cent,  to  40  per  cent,  of  the  stroke.  With  two  eccentrics, 
one  to  operate  the  admission  valves  and  the  other  to  operate  the 
exhaust  valves,  the  range  of  cut-off  may  be  extended  considerably 
beyond  mid-stroke. 

With  a  single  eccentric  valve  gear  the  admission  valve  must 
be  unhooked  for  cut-off  before  the  eccentric  reaches  the  end  of  its 


CORLISS  VALVE  GEARS 


201 


throw.  When  the  eccentric  reaches  the  end  of  its  throw  it 
begins  to  move  in  the  opposite  direction.  If  the  steam  valve, 
which  is  hooked  up  at  the  beginning  of  the  piston  stroke,  has 
not  been  released  by  this  time  it  will  not  be  released  at  all  but 
will  be  closed  by  the  gradual  motion  of  the  eccentric  on  its  return 
stroke.  Since  the  eccentric  is  set  a  little  more  than  90°  ahead  of 
the  crank  it  will  reach  the  end  of  its  stroke  before  the  crank 
reaches  a  vertical  position,  representing  mid-stroke  of  the  piston. 
This  will  be  made  clear  by  referring  to  Fig.  128. 

The  admission  valve  is  hooked  up  when  the  eccentric  is  in 
the  position  OC,  at  the  beginning  of  its  stroke.  When  the  crank 
reaches  its  dead  center  position  the  eccentric  is  in  the  position  OE 


FIG.  129. 

and  the  valve  has  been  opened  an  amount  equal  to  the  lead.  The 
valve  may  be  tripped  by  the  knock-off  cam  at  any  time  while  the 
eccentric  is  moving  through  the  angle  between  E  and  Ef.  Dur- 
ing this  time  the  crank  is  moving  from  the  position  OC  to  the 
position  OC'.  This  is  somewhat  less  than  half  the  piston  stroke. 
If  the  steam  valve  is  not  tripped  by  the  time  the  eccentric  reaches 
the  position  OE'  (and  the  crank  reaches  the  position  OC'}  it 
will  not  be  tripped  at  all,  but  will  remain  hooked  up  and  it  will 
then  be  closed  by  gradual  motion  of  the  eccentric  on  its  return 
stroke. 

By  referring  to  Fig.  128  it  will  be  seen  that  if  the  angle  of 
advance  is  increased  the  range  of  cut-off  will  be  decreased,  hence 
it  might  be  thought  that  a  greater  range  of  cut-off  could  be 


202 


STEAM  ENGINES 


secured  by  decreasing  the  angle  of  advance.  The  proper  working 
of  the  exhaust  valves  which  are  connected  with  the  eccentric  at 
all  times,  however,  sets  a  limit  to  the  decrease  in  the  angle  of 
advance. 

The  exhaust  valves,  being  connected  with  the  eccentric  at 
all  times,  have  the  same  motion  as  a  plain  slide  valve  and  this  mo- 
tion may  be  shown  by  a  Zeuner  valve  diagram.  Fig.  129  knows  a 
Zeuner  valve  diagram  for  the  head  end  exhaust  valve  of  a  Corliss 
engine,  the  eccentric  being  set  with  the  angle  of  advance  AOB. 
Release  occurs  when  the  crank  is  in  the  position  OR,  the  piston 
being  near  the  end  of  its  stroke  from  left  to  right.  Compression 
occurs  when  the  crank  is  in  the  position  OK,  the  piston  being  near 
the  end  of  its  stroke  from  right  to  left.  The  Zeuner  valve  diagram 
does  not  show  the  position  of  the  cut-off  for  a  Corliss  valve  gear 


FIG.  130. 

hence  it  is  of  but  little  use  in  setting  these  valves.  Corliss  valves 
are  usually  set  by  measurement  and  the  setting  then  checked 
from  an  indicator  diagram. 

Setting  Corliss  Valves. — When  the  various  parts  of  a  Corliss 
valve  gear  are  in  proper  adjustment  the  reach  rod  and  the 
eccentric  rod  should  be  of  such  length  that  both  the  rocker  arm 
and  the  wrist  plate  will  be  plumb  when  the  eccentric  is  vertical, 
as  shown  in  Fig.  130. 

Since  it  is  difficult  to  judge  by  the  eye  when  an  eccentric 
is  vertical  the  following  method  should  be  used  for  finding  exactly 
its  vertical  position.  A  tram  shaped  as  shown  at  A  in  Fig.  131 
is  made  of  sheet  iron  and  a  hole  bored  in  it  large  enough  to  receive 
a  scratch  awl  or  pointed  nail.  With  the  crotch  of  the  tram  placed 
against  the  shaft  and  with  a  scratch  awl  in  the  hole  in  the  tram 


CORLISS  VALVE  GEARS 


203 


the  arcs  BC  and  EF  are  drawn  on  the  eccentric  ending  at  the 
points  B  and  E  at  the  e$gfc  of  the  eccentric.  It  is  convenient 
to  rub  chalk  on  the  eccentric  and  the  arcs  drawn  in  the  chalk 
in  order  to  make  them  more  easily  seen.  With  a  pair  of  dividers 
draw  arcs  from  B  and  E  which  meet  exactly  on  the  edge  of  the 
eccentric  as  at  G.  By  a  similar  method  the  point  7  exactly 
opposite  G  is  located.  The  point  G  is  furthest  away  from  and 
the  point  /  is  nearest  the  center  of  the  shaft,  therefore  the  line 
IG  represents  the  position  of  the  eccentric. 

The  joint  in  the  eccentric  strap  is  usually  at  right  angles  to 
the  eccentric  rod,  hence  the  eccentric  may  be  placed  in  a 
vertical  position  by  turning  it  until  the  point  G  comes  to  the 


FIG.  131. 

joint.  In  case  the  joint  in  the  eccentric  strap  is  not  at  right 
angles  to  the  eccentric  rod,  the  following  method  of  placing  the 
eccentric  in  a  vertical  position  may  be  used. 

Another  tram,  shaped  as  shown  at  H,  Fig.  132,  is  made  of  steel 
wire.  A  punch  mark  J  is  made  on  the  side  of  the  eccentric  rod 
and  on  its  center  line.  With  one  end  of  the  tram  in  the  mark  J 
arcs  are  drawn  on  the  eccentric  strap  ending  at  the  points  K  and 
L  on  the  edge  of  the  strap.  With  a  pair  of  dividers  draw  arcs 
M  from  K  and  L  which  meet  exactly  on  the  edge  of  the  strap. 
When  the  point  M  coincides  with  either  the  points  G  or  7  the 
eccentric  is  exactly  on  center. 

With  the  dividers  set  to  the  length  KM ,  strike  arcs  P  and  R 
from  7  which  end  at  the  edge  of  the  strap.  Draw  arcs  S  from  P 


204 


STEAM  ENGINES 


and  K  which  meet  at  the  edge  of  the  strap  and  arcs  T  from  R  and 
L  which  meet  at  the  edge  of  the  strap.  When  the  point  G  coin- 
cides with  either  the  points  S  or  T  the  eccentric  is  exactly  vertical. 


FIG.  132. 


The  manufacturers  of  Corliss  engines  usually  place  a  mark  A, 
Fig.  133,  on  the  hub  of  the  wrist  plate  and  three  marks  D,  B,  and 
C  on  the  wrist  plate  stud.  When  A  coincides  with  B  the  wrist 


(L  Ji\ 

FIG.  133. 

plate  is  in  its  central  position,  and  in  this  position  the  rocker 
arm  and  eccentric  should  be  vertical,  and  if  they  are  not  vertical 
the  length  of  the  eccentric  and  reach  rods  should  be  adjusted 


CORLISS  VALVE  GEARS  205 

until  they  are  vertical.  When  A  coincides  with  D  the  wrist 
plate  is  at  one  extreme  of  \ts  travel  and  the  eccentric  is  on  dead 
center;  and  when  A  coincides  with  C  the  wrist  plate  is  at  the 
other  extreme  of  its  travel  and  the  eccentric  is  on  the  other  dead 
center.  If  the  marks  A,  B,  C,  and  D  are  not  on  the  wrist  plate 
and  stud  they  should  be  placed  on  with  a  chisel,  A  and  B  being 
marked  when  the  eccentric  is  vertical  and  the  rocker  arm  and 
wrist  plate  are  plumb,  and  D  and  C  being  marked  opposite  A 
when  the  eccentric  is  on  its  dead  centers. 

The  reach  rod  is  now  disconnected  from  the  wrist  plate  and  the 
wrist  plate  placed  on  its  central  position  (so  that  A  coincides  with 
B,  Fig.  133).  With  the  wrist  plate  in  its  central  position  and  both 


FIG.  134. 

steam  valves  hooked  on,  the  valves  should  have  the  proper  lap. 
The  amount  of  the  lap  may  be  measured  by  removing  the  steam 
bonnets  on  the  side  of  the  cylinder  opposite  the  wrist  plate  and 
inspecting  the  marks  on  the  valve  and  cylinder.  These  marks, 
as  shown  in  Fig.  134,  are  placed  on  by  the  manufacturer  of  the 
engine,  the  mark  F  on  the  valve  being  opposite  its  working  edge, 
and  Ej  on  the  cylinder  being  opposite  the  edge  of  the  port.  When 
F  coincides  with  E  the  edge  of  the  valve  is  exactly  in  line  with 
the  edge  of  the  port.  By  removing  the  valve  it  can  be  deter- 
mined which  edge  of  the  port  the  mark  E  on  the  cylinder  is 
opposite  and  therefore  it  will  be  known  on  which  side  of  E  the 
mark  F  should  be  for  the  valve  to  have  lap.  The  lap  is  measured 
with  a  pair  of  dividers,  being  the  distance  between  E  and  F, 


206 


STEAM  ENGINES 


Fig.  134.  With  the  wrist  plate  in  its  central  position  the  laps 
on  the  two  ends  of  the  cylinder  should  be  equal.  The  laps  may 
be  adjusted  by  lengthening  or  shortening  the  radial  rods,  which 
are  provided  with  left-  and  right-hand  thread  connections  for 
this  purpose.  The  proper  amount  of  lap  to  give  the  steam  valves 
depends  upon  the  size  of  cylinder  and  may  be  determined  from 
the  following  table: 

TABLE  FOR  SETTING  CORLISS  VALVES 


Diam.  of  cylinder, 
inches  • 

Lap  of  steam  valves, 
inches 

Lap  of  exhaust 
valves,  inches 

Lead  of  steam 
valves 

8 

Me 

Me 

l/32 

10 

Me 

Me 

yS2 

12 

Me 

Me 

y*2 

14 

H 

H 

y*2 

16 

H 

H 

y32 

18 

H 

y* 

y32 

20 

H 

y* 

y32 

22 

Me 

Me 

H* 

24 

Me 

Me 

K* 

26 

Me 

Me 

K* 

28 

Me 

Me 

H* 

30 

& 

Me 

%4 

32 

H 

H 

Me 

34 

H 

M 

Me 

36 

H 

K 

Me 

The  exhaust  valves  are  now  given  equal  laps  by  adjusting  the 
length  of  their  radial  rods  in  the  same  manner  as  was  done  with 
the  steam  valves.  Marks  are  placed  by  the  manufacturer  on  the 
cylinder  and  on  exhaust  valves,  similar  to  those  on  the  steam 
valves,  and  the  exhaust  laps  may  be  set  by  measurement  between 
these  marks.  The  amount  of  the  exhaust  lap  is  determined  from 
the  above  table. 

The  wrist  plate  is  now  turned  to  its  head  end  extreme  position 
and  the  length  of  the  dashpot  rod  adjusted  so  there  will  be  equal 
clearance  around  the  catch  block  as  shown  in  Fig.  135,  at  G  and 
H.  It  is  very  important  to  adjust  the  length  of  the  dashpot 
rods  properly  because  if  they  are  made  too  short  the  valves  will 
not  hook  on  and  if  they  are  too  long  the  valve  stem  is  liable  to  be 
bent  or  the  steam  bracket  broken,  or  both.  Now  turn  the  wrist 
plate  to  its  crank  end  extreme  position  and  adjust  the  length  of 


CORLISS  VALVE  GEARS 


207 


the  crank  end  dashpot  rod  as  was  done  for  the  head  end  dashpot 
rod.  V  ^.f 

The  steam  valves  may  nqw  be  given  their  proper  lead,  as  in- 
dicated by  the  above  table.  To  do  this  the  engine  is  placed  on 
its  head  end  dead  center,  using  a  tram  to  locate  the  dead  center 
exactly.  The  eccentric  is  then  loosened  on  the  shaft  and  the 
reach  rod  hooked  to  the  wrist  plate.  The  wrist  plate  is  then 
moved  over  to  its  head  end  extreme  position  in  order  to  hook 
up  the  head  end  steam  valve.  The  eccentric  is  then  turned 
around  on  the  shaft  until  the  por\  is  open  the  amount  of  the 


FIG.   135. 


desired  lead.  The  eccentric  is  then  fastened  in  this  position. 
The  port  opening  may  be  measured  by  the  lines  on  the  valve  and 
back  side  of  the  cylinder.  The  engine  should  now  be  turned 
to  the  crank  end  dead  center,  the  crank  end  steam  valves  hooked 
up,  and  the  lead  measured  to  see  if  it  is  equal  to  that  on  the  head 
end.  If  it  is  not,  any  slight  adjustment  that  may  be  required 
can  be  made  by  moving  the  eccentric. 

The  governor  and  governor  rods  should  next  be  adjusted  if 
they  require  it.     Fig.  136  shows  a  common  form  of  Corliss  engine 


208 


STEAM  ENGINES 


governor  with  its  connections  to  the  valves,  parts  of  the  governor 
being  cut  away  to  show  its  construction.  As  shown  here  the 
parts  of  the  governor  are  in  the  position  they  will  occupy  when 


the  engine  is  running  at  its  normal  speed.  If  the  speed  rises 
above  normal,  centrifugal  force  throws  the  governor  balls 
farther  from  the  center.  This  raises  the  cross  bar  and  with  it 
the  drop  rod  arm  and  throws  the  knock-off  levers  (on  the  valve 


CORLISS  VALVE  GEARS  209 

stems)  around  so  that  the  knock-off  cams  strike  the  steam  hook 
earlier  and  thus  cause  an  earlier  cut-off.  If  the  speed  falls  below 
normal  the  knock-off  cams  $re  moved  in  the  opposite  direction 
and  cut-off  occurs  later. 

If  the  belt  which  runs  the  governor  should  break,  the  cross  bar 
would  drop  to  its  lowest  position  and  this  would  make  cut-off 
come  at  the  latest  possible  point  in  the  stroke  or  the  steam  hooks 
would  not  be  disengaged  at  all  and  the  cylinder  would  take  steam 
for  the  entire  stroke.  This  would  cause  the  engine  to  run 
away.  * 

In  order  to  prevent  the  engine  from  running  away  if  the 
governor  belt  breaks  or  any  other  accident  happens  which  would 
throw  the  governor  to  its  lowest  position,  safety  cams  are  placed 
on  the  knock-off  levers.  When  the  governor  falls  to  its  lowest 
position  the  knock-off  levers  are  thrown  around  far  enough  to 
bring  the  safety  cams  under  the  steam  hooks,  thus  preventing 
the  admission  of  any  steam  to  the  cylinder.  If  the  safety  cams 
are  allowed  to  %come  into  action  as  described  above  the  engine 
could  not  be  started,  after  it  was  shut  down,  until  the  governor 
was  raised  far  enough  to  prevent  the  safety  cams  from  coming 
in  contact  with  the  steam  hooks.  It  would  be  a  nuisance  to  have 
to  raise  the  governor  every  time  the  engine  is  started,  hence  a 
governor  safety  stop  is  placed  on  the  governor  to  prevent  it 
from  falling  to  its  lowest  position  and  bringing  the  safety  cams 
into  action  when  the  engine  is  shut  down  by  hand. 

When  the  engine  is  to  be  shut  down  by  hand  the  governor 
safety  stop  is  raised  to  the  position  shown  in  Fig.  136.  The  cross 
bar  will  then  rest  on  the  safety  stop,  which  is  high  enough  above 
its  lowest  position  to  prevent  the  safety  cams  from  coming  into 
operation.  In  this  position  of  the  cross  bar  the  valves  will  hook 
up  and  open  when  steam  is  turned  on  again.  As  soon  as  the 
engine  is  started  again  the  safety  stop  falls  to  one  side.  If,  then, 
the  governor  belt  should  break  the  cross  bar  would  fall  to  its 
lowest  position  and  bring  the  safety  cams 'into  operation. 

When  the  engine  is  running  at  normal  speed  the  cross  bar  on 
the  governor  is  about  halfway  between  the  upper  limit  of  its 
travel  and  the  end  of  the  safety  stop  (in  its  raised  position), 
hence  in  adjusting  the  governor  it  should  be  blocked  up  to  this 
position.  The  length  of  the  governor  drop  rod  is  then  adjusted 
until  the  drop  rod  arm  is  horizontal  and  the  bell  crank  stands 
vertically.  The  governor  is  then  unblocked,  the  engine  started 


210  STEAM  ENGINES 

slowly,  and  the  length  of  the  governor  rods  adjusted  so  that 
cut-off  is  equal  on  both  ends  of  the  cylinder.  The  governor  rods 
are  provided  with  right-  and  left-hand  screws  so  their  length  may 
be  changed  without  stopping  the  engine.  An  indicator  diagram 
should  be  used  to  determine  when  the  cut-off  is  equal  on  the  two 
ends,  as  well  as  for  all  other  valve  adjustments. 


CHAPTER  XVI 
GOVERNING 

Governing. — The  work  that  most  steam  engines  do  requires  a 
constant  or  practically  constant  speed  of  rotation.  This  require- 
ment is  more  difficult  to  meet  than  might  at  first  appear,  and 
much  thought  has  been  expended  on  this  problem  in  order  to 
solve  it  satisfactorily. 

As  mentioned  in  a  previous  chapter  changes  of  speed  occur  in 
two  entirely  different  ways.  First,  the  speed  may  change  during 
a  single  revolution  or  cycle  of  the  engine  due  to  a  variation  in 
pressure  against  the  piston  and  to  the  angle  at  which  the  force 
of  the  steam  pressure  is  transmitted  to  the  crank.  Second, 
the  speed  may  change  due  to  a  change  of  load  or  to  varying 
boiler  pressure,  such  a  change  extending  over  a  period  of  more 
than  one  revolution.  The  first  of  these  kinds  of  speed  variation 
is  taken  care  of  by  the  flywheel  which  stores  up  energy  during 
one  part  of  a  revolution  and  gives  it  out  again  during  another 
part,  as  explained  in  a  previous  chapter.  The  second  kind  of 
speed  variation  must,  however,  be  corrected  by  some  kind  of 
controlling  device,  or  governor. 

If  the  load  on  an  engine  is  increased  or  if  the  boiler  pressure 
of  the  steam  becomes  less  the  speed  of  the  engine  will  decrease. 
On  the  other  hand,  if  the  load  decreases  or  the  boiler  pressure  in- 
creases the  speed  of  the  engine  will  increase.  The  governor  is  for 
the  purpose  of  regulating  the  supply  of  steam  to  the  engine  so  that 
its  speed  will  remain  constant  or  practically  constant.  In  order 
to  do  this  a  steam  engine  governor  either  operates  on  a  throttle 
valve  placed  between  the  engine  and  boiler  to  change  the  pressure 
of  the  steam  which  is  being  admitted  to  the  engine;  or  it  alters 
the  volume  of  steam  admitted  to  the  engine  by  changing  the 
point  of  cut-off. 

Whatever  method  of  controlling  the  speed  is  used,  no  governor 
can  control  the  speed  perfectly  because  the  governor  is  run  by  the 
engine  itself  and  some  change  in  speed  must  occur  before  the 

211 


212  STEAM  ENGINES 

governor  can  operate.  That  is,  the  governor  operates  on  account 
of  a  change  of  speed,  hence  the  governor  cannot  keep  the  speed 
of  the  engine  absolutely  constant.  Also,  any  change  of  speed 
which  occurs  after  the  steam  has  passed  beyond  the  influence  of 
the  governor  cannot  be  controlled  until  the  next  stroke  of  the  pis- 
ton. For  example,  if  the  load  changes  after  cut-off  has  occurred 
this  may  affect  the  speed  of  the  engine  and  the  governor  can- 
not have  any. effect  because  it  can  only  operate  on  the  next 
admission  of  steam  or  during  the  next  stroke  of  the  piston.  This 
is  especially  true  of  compound  engines,  where  the  governor 
controls  the  supply  of  steam  to  the  high  pressure  cylinder  only. 
If  a  change  of  load  occurs  after  cut-off  in  the  high  pressure  cylin- 
der, the  steam  in  this  cylinder  expands  and  does  work  in  the  cylin- 
der and  then  passes  into  the  low  pressure  cylinder  and  again 
expands  and  does  work,  all  outside  of  the  control  of  the  governor, 
which  can  only  act  upon  the  next  admission  to  the  high  pressure 
cylinder.  But  for  all  of  the  difficulties  in  the  way  of  securing 
close  speed  regulation,  a  good  steam  engine  governor  will  control 
the  speed  within  2  per  cent,  of  its  normal  speed. 

Pendulum  Governor. — Nearly  all  steam  engine  governors 
operate  through  centrifugal  force.  They  usually  consist  of  a 
pair  of  weights  revolving  about  a  spindle  which  is  driven  from  the 
engine  shaft.  The  centrifugal  force  of  the  revolving  weights  is 
resisted  by  some  controlling  force,  such  as  gravity,  the  tension 
of  a  spring,  or  both.  When  the  engine  (and  governor)  is  running 
at  constant  speed  the  weights  take  up  a  fixed  position  at  which  the 
controlling  force  just  balances  the  centrifugal  force.  When  an 
increase  of  speed  occurs  the  additional  centrifugal  force  causes 
the  weights  to  move  outward  to  a  new  position,  and  in  moving 
outward  they  act  upon  the  throttle  valve  or  some  form  of  auto- 
matic gear  by  which  the  cut-off  is  varied  so  that  the  speed  is 
reduced. 

The  most  common  forms  of  centrifugal  governors  are  those 
known  as  pendulum  governors.  In  these  governors  the  spindle  is 
vertical  and  there  are  two  weights,  each  of  which  is  placed  at  the 
end  of  an  arm.  The  two  arms  are  suspended  from  the  top  of  the 
spindle  and  pivoted  at  or  near  it.  When  the  spindle  is  rotated 
the  weights  move  outward  and  upward  and  their  upward  motion 
is  resisted  by  the  force  of  gravity..  When  the  engine  is  running 
at  constant  speed  the  weights  take  up  a  position  in  which  the 
force  of  gravity,  or  other  controlling  force,  just  balances  the  cen- 


GOVERNING 


213 


trifugal  force.  If  the  speed  of  the  engine  is  increased  or  decreased 
the  weights  take  up  a  new -position  in  which  the  controlling 
force  balances  the  centrifugal  force  developed  by  the  revolving 
weights. 

A  form  of  governor  commonly  used  with  plain  slide  valve 
engines  and  operating  on  the  above  principles  is  illustrated  in 


FIG.  137. 

Fig.  137.  This  is  a  throttling  governor  since  it  acts  upon  a 
throttle  valve  and  thus  controls  the  pressure  of  the  steam  ad- 
mitted to  the  cylinder.  In  the  governor  illustrated  here  the 
opening  A  is  connected  to  the  steam  chest  and  the  opening  B 
connected  to  the  steam  pipe  leading  from  the  boiler,  so  that  the 
steam  passes  through  the  valve  C  before  entering  the  cylinder. 
The  governor  is  run  by  a  belt  from  the  engine  shaft  to  the  pulley 


214  STEAM  ENGINES 

E.  This  transmits  motion  through  the  gear  wheels  D  and  F 
to  the  vertical  spindle  to  which  the  weights  G  and  G  are  attached. 
As  the  speed  of  the  engine  increases  the  weights  move  outward 
and  upward  and  press  downward  upon  the  valve  stem  H  thus 
partly  closing  the  valve  C.  In  the  same  way,  a  decrease  in  the 
speed  of  the  engine  causes  the  weights  to  assume  a  lower  position 
and  the  valve  stem  H  rises  and  admits  more  steam  to  the  cylinder, 
thus  causing  an  increase  of  speed. 

The  upward  movement  of  the  weights  is  resisted  partly  by  the 
force  of  gravity  and  partly  by  the  tension  of  the  spring  K,  so  that 
for  any  particular  speed  of  the  engine  the  weights  will  take  a 
position  at  which  their  centrifugal  force  is  just  balanced  by  the 
force  of  gravity  and  the  tension  of  the  spring.  If  the  engine 
departs  from  this  speed  the  weights  will  rise  or  fall  until  the 
steam  pressure  is  adjusted  to  suit  the  load  which  the  engine  is 
carrying. 

The  speed  of  the  engine  can  be  changed  by  changing  the 
tension  of  the  spring  K  by  means  of  adjusting  screw  T.  If  the 
tension  of  the  spring  is  increased,  the  weights  will  have  to  revolve 
faster  in  order  to  secure  a  given  opening  of  the  throttle  valve, 
and  thus  the  speed  of  the  engine  will  be  increased.  In  the  same 
way,  the  speed  of  the  engine  may  be  reduced  by  decreasing  the 
tension  of  the  spring. 

The  form  of  governor  described  above  was  invented  by  James 
Watt,  one  of  the  early  inventors  of  the  steam  engine,  and  for  this 
reason  it  is  sometimes  called  the  Watt  governor.  It  is  one  of 
the  simplest  forms  of  steam  engine  governors  used  at  the  present 
time. 

Stability. — A  governor  is  said  to  be  stable  when  there  is  a 
definite  position  of  the  weights  for  any  definite  speed;  that  is, 
if  the  speed  of  the  engine  changes  by  any  amount  the  weights 
move  up  or  down  to  a  new  position  which  corresponds  to  that 
particular  speed,  and  then  remain  in  this  new  position  until 
there  is  another  change  of  speed.  From  the  preceding  descrip- 
tion of  the  Watt  governor  it  will  be  seen  that  the  new  position 
of  the  weights  gives  a  larger  or  smaller  opening  of  the  throttle 
valve  and  this  serves  to  bring  the  speed  back  to  normal.  The 
speed  is  thus  automatically  maintained  at  or  near  the  number  of 
r.p.m.  for  which  the  governor  is  set. 

If  a  governor  was  unstable  it  would  have  no  definite  position 
for  a  given  speed  and  its  movements  would  be  irregular  and  un- 


GOVERNING 


215 


certain.  For  this  reason  it  could  not  maintain  a  constant  speed 
of  the  engine  and  would  therefore  not  be  suitable  for  governing 
a  steam  engine  from  which  a^onstant  speed  was  desired. 

It  is  evident  from  the  above  discussion  that  stability  is  a 
desirable  and  even  necessary  quality  of  a  steam  engine  governor 
because  a  stable  governor  is  always  in  equilibrium  and  exercises 
a  positive  control  over  the  speed. 

In  order  for  a  governor  to  be  stable  the  controlling  force,  or 
the  force  acting  against  the  rise  of  the  weights,  must  increase  at 
a  faster  rate  than  the  radius  of  the  circle  about  which  the  weights 
are  revolving,  and  the  larger  this  ratio  between  the  controlling 
force  and  the  radius  of  the  circle  about  which  the  weights  are 
revolving,  the  greater  will  be  the  stability  of  the  governor. 


FIG.  138. 

A  change  in  the  speed  of  a  governor  causes  a  change  in  the 
position  of  the  weights,  and  if  the  governor  is  stable  there  is 
only  one  position  of  the  weights  which  correspond  with  a  given 
speed.  Since  the  steam  supply  depends  upon  the  position  of 
the  weights,  a  stable  governor  cannot  maintain  a  strictly  con- 
stant speed,  because  if  the  boiler  pressure  or  load  changes  a  cer- 
tain displacement  of  the  weights  s  necessary  to  admit  more  or 
less  steam  and  the  weights  can  maintain  this  new  position  only 
by  turning  faster  or  slower.  However,  the  variaton  from  a 
constant  speed  can  be  reduced  by  reducing  the  stability  of  the 
governor. 

The  ordinary  forms  of  pendulum  governors  such  as  illustrated 
in  Fig.  138  are  stable  and  also  the  crossed  arm  form  of  pendulum 
governor  illustrated  in  Fig.  139  provided  the  points  of  support 
are  located  close  to  the  central  column. 

Sensibility. — The  movement  of  the  weights  of  a  governor  from 
their  lowest  to  their  highest  position  can  produce  only  a  certain 
21 


216 


STEAM  ENGINES 


movement  of  the  regulating  mechanism,  whether  it  is  a  throttle 
valve  or  a  cut-off  attachment.  This  will  produce  the  greatest 
change  of  speed  for  which  the  governor  is  responsible.  If  an 
engine  is  overloaded  or  if  the  steam  pressure  is  too  low,  the  speed 
may  drop  even  after  the  governor  has  done  all  that  it  can  do  to 
admit  steam  freely,  but  the  variation  in  speed  for  which  the 
governor  is  responsible  is  only  that  which  will  cause  the  weights 
to  move  from  the  position  of  no  steam  to  the  position  of  full 
steam.  When  a  small  variation  of  speed  is  sufficient  to  do  this 
the  governor  is  said  to  be  sensitive. 

It  is  evident  from  the  above  discussion  that  the  more  sensitive 
a  governor  is  the  less  stable  it  must  be.  As  both  of  these  features 
can  be  controlled  in  the  design  of  the  governing  mechanism,  the 


FIG.  139. 

designer  aims  at  securing  a  governor  which  is  stable  and  which  at 
the  same  time  is  sensitive  enough  to  control  the  speed  within 
the  limits  needed  for  the  kind  of  work  the  engine  is  to  do. 

It  is  absolutely  necessary  that  a  steam  engine  governor  be 
stable  and  it  is  highly  desirable  that  it  be  sensitive.  However, 
it  should  riot  be  too  sensitive  as  this  causes  the  engine  to  hunt 
or  over-govern.  Hunting  is  brought  about  by  the  conditions 
mentioned  below. 

When  an  alteration  of  speed  begins,  the  governor  does  not  act 
immediately  because  the  governor  can  only  operate  after  a 
change  of  speed  has  occurred.  Moreover,  a  change  in  position 
of  the  governor  does  not  affect  the  speed  of  the  engine  immedi- 
ately both  on  account  of  the  inertia  of  the  moving  parts  of  the 
engine  which  has  the  effect  of  resisting  a  change  of  speed,  and 
also  because  of  the  energy  contained  in  the  steam  which  has 
passed  the  control  of  the  governor.  If  the  governor  is  of  the 


GOVERNING 


217 


throttling  type,  the  steam  chest  is  filled  with  steam  which  has 
passed  the  control  of  the  governor  at  the  time  the  change  of 
speed  begins,  and  if  the  governor  acts  upon  the  cut-off  its  oppor- 
tunity for  controlling  the  speed  has  passed  if  cut-off  has  occurred. 
Hence,  there  is  a  certain  time  lag  between  the  governor  and  the 
engine  speed.  The  consequence  of  this  is  that,  if  the  governor 
is  too  sensitive,  by  the  time  the  change  in  engine  speed  has  had 


Fio.   140. 

full  effect  upon  the  governor,  it  is  forced  into  a  position  of  over- 
control  or  a  position  beyond  that  which  is  necessary  to  bring 
the  engine  speed  back  to  normal.  The  speed  of  the  engine  then 
begins  to  change  in  the  opposite  direction  and,  for  the  same 
reasons,  the  governor  is  forced  into  a  position  of  over-control 
in  the  opposite  direction.  Thus  a  state  of  forced  oscillation 
is  set  up  which  causes  the  speed  to  be  first  too  high  and  then  too 
low,  a  condition  known  as  hunting. 


218 


STEAM  ENGINES 


Hunting  is  avoided  by  allowing  a  certain  margin  of  stability 
in  the  governor,  that  is,  by  not  making  it  too  sensitive,  and  also  by 
the  use  of  dashpots  attached  to  the  governor  in  such  way  as  to 
dampen  its  motion  in  case  it  is  too  sensitive. 

Some  governors,  on  account  of  their  form,  are  much  more 
sensitive  than  others.  It  has  been  found  that  if  the  form  of 
governor  is  such  that  the  weights,  in  rising,  follow  a  parabola 
instead  of  a  circle  the  governor  will  be  extremely  sensitive.  A 
governor  constructed  in  this  way,  illustrated  in  Fig.  140,  is  so 
sensitive  that  an  air  cylinder  and  piston  is  placed  at  the  top  of 


FIG.  141. 

the  column  to  check  the  movement  of  the  weights.  Other  forms 
of  sensitive  governors  are  the  Proll  governor  illustrated  in  Fig.  141 
and  the  Hartnell  governor  illustrated  in  Fig.  142.  In  the  Hart- 
nell  governor,  the  weights  move  in  a  practically  horizontal  path 
and  the  controlling  force  is  furnished  by  a  coil  spring  shown  at 
the  top  of  the  central  column,  and  the  sensitiveness  of  the 
governor  can  be  adjusted  by  means  of  this  spring.  This  form  of 
governor  is  more  suitable  for  high  speed  engines  than  for  low 
speed  ones  on  account  of  the  small  size  of  the  weights. 

The  Proll  governor  illustrated  in  Fig.  141  is  better  adapted  to 
slow  speed  engines,  such  as  the  Corliss  engine.  This  type  of 
governor  is  known  as  a  loaded  governor  on  account  of  the  heavy 


GOVERNING 


219 


weight  placed  around  the  central  column.  This  weight  revolves 
with  the  governor  and  has  the  effect  of  increasing  the  controlling 
force  without  adding  to  th*  centrifugal  force,  as  would  be  the 
case  if  the  additional  weight  was  placed  at  the  ends  of  the  rotating 
arms. 

The  advantages  of  a  loaded  governor  are  that  it  is  more 
powerful  than  an  unloaded  one,  that  the  increase  in  power  is 
gained  without  a  corresponding  loss  of  sensitiveness,  and  that  it 


FIG.  142. 

may  be  run  at  a  lower  speed  than  an  unloaded  governor  having 
balls  of  equal  size. 

A  powerful  governor  is  necessary  in  order  to  overcome  the 
friction  of  the  moving  parts  of  the  governor  and  controlling 
mechanism.  Friction  in  a  governor  and  its  connected  mechanism 
has  the  effect  of  increasing  the  controlling  force  and  thus  reducing 
the  sensitiveness  of  the  governor.  If  the  controlling  force  of  a 
governor,  neglecting  friction,  is  represented  by  F  and  the  force 
necessary  to  overcome  friction  is  represented  by  /  then  for  an 
increase  of  speed  the  centrifugal  force  acting  on  the  weights 
must  be  increased  to  F  +  /  in  order  to  change  the  position  of  the 


220 


STEAM  ENGINES 


governor  and  also  the  centrifugal  force  must  be  decreased  to  F  — 
/  in  order  to  change  the  ptisition  of  the  governor  for  a  decreasing 
speed. 

A  loaded  governor  of  the  type  shown  in  Fig.  141  or  Fig.  136, 
that  is,  a  governor  having  four  arms,  has  the  further  advantage 
that  the  vertical  movement  of  the  collar,  or  central  column, 
is  twice  as  great  as  the  movement  of  the  weights  at  the  ends  of 
the  rotating  arms.  That  is,  for  a  given  change  of  speed  a  gover- 
nor of  the  types  shown  in  Figs.  136  and  141  produces  twice  as 
much  motion  in  the  controlling  mechanism  as  in  the  plain  pendu- 
lum governor  such  as  illustrated  in  Fig.  138a.  The  result  of 


FIG.  143. 

this  is  that  these  governors  may  be  run  at  a  less  speed  than  the 
Watt  governor,  hence  their  general  use  on  low  speed  engines  of 
the  Corliss  type. 

Shaft  Governors. — Governors  which  are  located  in  the  fly- 
wheel and  which  turn  with  the  flywheel  are  commonly  called 
shaft  governors.  These  are  usually  attached  to  the  spokes  of 
the  flywheel  and  operate  by  shifting  the  position  of  the  eccentric 
and  thus  changing  the  point  of  cut-off  so  the  amount  of  steam 
admitted  to  the  cylinder  is  in  proportion  to  the  load  which  the 
engine  carries. 


GOVERNING 


221 


One  example  of  shaft  governor  operating  on  this  principle  is 
described  in  Chapter  13  and  illustrated  in  Fig.  108.  The  prin- 
ciples upon  which  a  govern^  of  this  type  operates  are  the  same 
as  those  upon  which  the  centrifugal  pendulum  governor  operates. 
The  similarity  of  operation  of  these  two  kinds  of  governors  may 
be  seen  by  a  study  of  Fig.  143.  An  arm  is  pivoted  at  some  point 
on  the  flywheel  and  to  the  end  of  the  arm  is  attached  a  weight  W, 
the  length  of  the  arm  being  A.  At  a  distance  A'  from  the  pivot  a 
spring  is  attached  to  the  arm  and  is  arranged  so  as  to  act  at 
practically  right  angles  to  the  arm.  The  centrifugal  force  of 
the  weight  will  be  balanced  by  the  pull  of  the  spring  in  the  same 
manner  as  gravity  balances  the  centrifugal  force  in  a  pendulum 
governor. 


FIG.  144. 


With  a  shaft  governor,  the  speed  at  which  the  engine  will  run 
increases  as  the  tension  in  the  spring  is  increased,  or  as  the 
distance  between  the  pivot  and  the  point  of  attachment  of  the 
spring  is  increased,  or  as  the  weight  at  the  end  of  the  arm  is  made 
less,  because  in  these  .cases  the  governor  must  rotate  faster  to 
maintain  the  same  position.  As  it  is  usually  impractical  to 
change  the  weight  of  the  arms,  the  speed  is  usually  changed  by 
changing  the  tension  of  the  spring.  For  this  purpose  the  spring 
is  provided  with  a  turnbuckle  or  some  other  arrangement  by 
which  its  tension  may  be  readily  changed. 

It  is  sometimes  desired  to  reverse  the  direction  of  rotation 
of  an  engine  which  is  fitted  with  a  shaft  governor.  This  can  be 


222 


STEAM  ENGINES 


done  with  some  engines  but  with  others  it  cannot  be  done  as  the 
manufacturer  has  not  made  provision  for  it. 

In  order  to  reverse  the  direction  of  rotation  of  a  slide  valve 
engine  the  eccentric  must  be  turned  through  an  angle  of  180 
degrees  on  the  shaft.  In  a  shaft  governor  the  eccentric  is  con- 
nected directly  to  the  arms  of  the  governor,  consequently  the 
arms  must  be  turned  around  so  as  to  swing  in  the  opposite 
direction  and  the  attachment  of  the  springs  to  the  rim  or  spokes 
of  the  flywheel  reversed  by  attaching  them  to  other  holes,  if 
these  have  been  provided  by  the  manufacturer.  The  changes 
required  in  order  to  reverse  the  direction  of  rotation  of  an  engine 
supplied  with  a  shaft  governor  are  illustrated  in  Fig.  144.  In 
this  illustration  (a)  shows  the  -arrangement  of  the  parts  for  one 


FIG.   145. 


direction  of  rotation  and  (6)  shows  the  arrangement  of  the  parts 
for  the  opposite  direction  of  rotation.  As  each  manufacturer 
of  high  speed  engines  has  his  own  arrangement  of  shaft  governor 
it  is  impossible  to  give  definite  directions  for  the  proper  arrange- 
ment of  the  governor  in  order  to  reverse  the  direction  of  rotation, 
consequently,  if  this  is  desired,  it  is  best. to  write  to  the  manu- 
facturer before  attempting  to  make  any  changes  in  the  governor. 
Inertia  Governor. — A  form  of  shaft  governor  invented  by  F. 
M.  Rites  and  known  as  the  Rites  inertia  governor  is  used  on  sev- 
eral makes  of  automatic  high  speed  engine.  In  this  governor, 
which  is  illustrated  in  Fig.  145,  a  heavy  bar  on  the  flywheel, 
carrying  two  weights  F  and  A,  swings  about  the  pin  B.  The 
eccentric,  which  is  usually  only  a  pin  located  near  the  center  of 


GOVERNING  223 

the  engine  shaft  is  carried  by  the  arm.  As  the  arm  swings  about 
the  pin  B,  the  eccentric  pin  C  swings  closer  to  or  further  away 
from  the  center  of  the  shafih;and  thus  changes  the  eccentricity. 
The  controlling  force  is  furnished  by  the  coil  spring  D,  which  has 
one  end  fastened  to  the  arm  of  the  governor  and  the  other  end 
fastened  to  a  spoke  of  the  flywheel. 

This  governor  operates  by  the  force  of  inertia,  or  the  tendency 
of  the  weights  to  keep  on  moving  at  a  constant  speed,  when  the 
speed  of  the  flywheel  changes.  If  the  engine  is  running  at  a 
constant  speed  the  flywheel  and  governor  weights  will  be  turning 
at  the  same  rate.  Referring  to  Fig.  145,  suppose  a  load  is  sud- 
denly put  on  the  engine.  This  slackens  the  speed  of  the  flywheel, 
but  the  inertia  of  the  governor  weights  causes  them  to  move 
forward  at  the  same  rate  as  before.  This  moves  the  eccentric 
pin  further  away  from  the  center  of  the  shaft,  which  increases 
the  eccentricity  and  causes  a  later  cut-off.  Also,  if  the  load 
should  be  decreased,  the  speed  of  the  flywheel  will  increase, 
causing  the  governor  weights  to  lag  behind  and  reduce  the 
eccentricity.  This  causes  cut-off  to  occur  earlier  and  bring  the 
speed  back  to  its  normal  value. 

The  inertia  governor  described  above  is  extremely  simple  but 
in  securing  this  simplicity  some  things  have  been  sacrificed.  One 
of  these  is  that  the  governor  does  not  give  a  constant  lead,  which 
is  desirable  for  a  constant  speed  engine.  It  will  be  seen  also 
that  the  governor  is  unbalanced,  since  it  is  pivoted  away  from  the 
center  of  the  shaft.  This  causes  the  arm  to  tend  to  fall  forward 
during  one-half  of  the  revolution  and  to  fall  backward  during  the 
other  half  of  the  revolution.  If  the  speed  is  high,  say  over  250 
revolutions  per  minute,  this  effect  is  not  noticeable,  but  for 
lower  speeds  it  will  affect  the  cut-off.  If  the  speed  is  reduced 
much  below  200  revolutions  per  minute,  this  unbalancing 
effect  becomes  noticeable  as  a  jerk  in  the  governor  action 
which  may  send  the  governor  arm  through  its  whole  range 
every  second  or  third  revolution. 

To  prevent  this  action,  a  drag  or  brake  spring  is  attached  to 
the  rim  of  the  flywheel  in  such  manner  as  to  bear  against  one  of 
the  weights  with  enough  force  to  prevent  sudden  swinging  but 
not  enough  to  prevent  the  governor  from  swinging  when  there  is 
a  change  in  load.  In  addition  to  this  dampening  spring  there  is 
also  a  spring  bumper  fastened  to  the  rim  of  the  flywheel  to  prevent 
the  arm  from  swinging  too  far  and  damaging  the  valve. 

22 


224  STEAM  ENGINES 

In  some  forms  of  inertia  governor  a  second  arm  is  placed 
parallel  with  the  one  carrying  the  weights  and  arranged  so  the 
whole  governor  will  be  balanced.  This  makes  the  governor  more 
complicated  but  makes  it  suitable  for  running  at  low  speed  and 
it  gives  the  same  sensitiveness  as  the  unbalanced  governor  at 
high  speeds. 


CHAPTER  XVII 
COMPOUND  ENGINES 

Compounding. — The  low  efficiency  of  the  steam  engine  shows 
that  a  large  part  of  the  heat  energy  supplied  to  it  is  not  turned 
into  useful  work,  but  is  lost  or  wasted.  Even  the  best  engines 
utilize  only  about  20  per  cent,  of  the  heat  supplied  to  them, 
leaving  about  80  per  cent,  to  be  accounted  for  by  the  various 
losses  incident  to  the  operation  of  the  engine.  Radiation  of 
heat  from  the  engine  and  the  friction  of  the  moving  parts  account 
for  only  a  small  part  of  the  loss.  A  much  larger  part  is  accounted 
for  by  the  heat  contained  in  the  exhaust  steam.  The  loss  from 
this  source  may  be  reduced  considerably  by  the  use  of  a  conden- 
ser, which  lowers  the  exhaust  pressure  and  makes  a  larger  propor- 
tion of  the  total  supply  of  heat  available  for  useful  work;  but, 
even  with  the  use  of  a  condenser,  the  loss  of  heat  in  the  exhaust 
is  considerable. 

For  a  long  time  after  the  steam  engine  was  invented,  the 
three  sources  of  loss  mentioned  above,  namely,  radiation,  friction, 
and  loss  of  heat  in  the  exhaust  were  thought  to  be  the  only  ones. 
It  was  discovered  later,  however,  that  another  serious  source  of 
loss  comes  from  the  interchange  of  heat  between  the  steam  and 
the  cylinder  walls,  which  results  in  condensation  of  steam  inside  the 
cylinder.  The  manner  in  which  cylinder  condensation  produces 
a  loss  has  been  fully  discussed  in  Chapter  8  and  the  student, 
should  review  this  chapter  at  this  time  in  order  to  understand 
more  fully  the  principles  underlying  the  compound  engine. 

Since  cylinder  condensation  produces  such  large  losses  in  the 
operation  of  steam  engines,  it  becomes  a  matter  of  considerable 
importance  to  understand  the  causes  of  cylinder  condensation 
and  the  means  employed  for  reducing  it.  The  principal  cause  of 
cylinder  condensation  is  the  large  range  of  temperature  to  which 
the  walls  of  the  cylinder  (including  head  and  piston)  are  subjected 
during  each  revolution  of  the  engine.  This  range  of  temperature 
is  due  to  the  expansion  of  the  steam  in  the  cylinder  from  the  high 
pressure  of  admission  to  the  relatively  low  pressure  of  the  exhaust. 
23  225 


226  STEAM  ENGINES 

The  variation  of  pressure,  and  therefore  the  range  of  tempera- 
ture, in  an  engine  cylinder  depends  upon  the  cut-off.  With  a 
fixed  exhaust  pressure  an  early  cut-off  will  produce  a  large  varia- 
tion of  pressure  during  expansion  and  a  late  cut-off  will  produce  a 
small  variation  of  pressure.  It  is  evident  that  an  early  cut-off 
is  necessary  to  the  economical  use  of  the  steam  because  this 
permits  the  expansive  force  of  the  steam  to  be  utilized  more 
fully  than  with  a  late  cut-off  and  small  expansion.  The  engine 
designer  is  therefore  confronted  with  two  opposing  conditions. 
On  the  one  hand,  an  early  cut-off  increases  the  losses  from  cylin- 
der condensation,  and  on  the  other  hand,  an  early  cut-off  is 
necessary  if  the  steam  is  to  be  expanded  through  its  full  range  and 
utilized  efficiently. 

One  of  the  means  most  commonly  employed  for  reducing  the 
losses  from  cylinder  condensation  is  to  divide  the  total  expansion 
of  the  steam  into  two  or  more  parts  and  to  perform  each  part  of 
the  expansion  in  a  separate  cylinder,  thereby  reducing  the  range 
of  temperature  in  each  cylinder.  This  is  called  compounding, 
and  engines  in  which  the  total  expansion  of  the  steam  is  divided 
between  two  cylinders  are  called  compound  engines. 

Since  the  losses  from  cylinder  condensation  increase  as  the 
total  range  of  pressure  through  which  the  steam  is  expanded 
increases,  the  number  of  parts  into  which  the  total  expansion 
should  be  divided,  in  compounding,  depends  upon  the  pressure 
of  the  steam  supplied  to  the  engine.  It  also  depends,  to  a  certain 
extent,  upon  the  kind  of  work  for  which  the  engine  is  intended. 
In  marine  work,  where  compounding  is  more  generally  practised 
than  in  stationary  work,  the  number  of  parts  into  which  the 
total  expansion  is  divided  for  different  boiler  or  admission  pres- 
sures is  about  as  follows : 

Simple  engines 30  to    70  Ib.  per  sq.  in.  gage 

Compound 80  to  120  Ib.  per  sq.  in.  gage 

Triple  expansion 140  to  180  Ib.  per  sq.  in.  gage 

Quadruple  expansion 200  to  250  Ib.  per  sq.  in.  gage 

In  stationary  work  there  is  a  tendency  to  divide  the  total 
expansion  of  the  steam  into  a  fewer  number  of  parts  and  to  use 
higher  pressures.  Compound  condensing  engines  are  often 
run  with  pressures  of  120  to  150  Ib.  per  sq.  in.  gage,  while  the 
compound  locomotive,  which  is  not  used  with  a  condenser,  is 


COMPOUND  ENGINES  227 

sometimes  supplied  with  steam  having  a  pressure  of  200  to  225 
Ib.  per  sq.  in.  gage. 

Expansion  of  Steam.  —  An  ideal  expansion  line  for  steam 
expanding  from  120  Ib.  per  sq.  in.  absolute  pressure  to  an  exhaust 
pressure  of  1.6  Ib.  per  sq.  in.  absolute  pressure  is  shown  in  Fig. 
146.  The  diagram  ABCDEFG  represents  an  ideal  indicator 
diagram  if  the  total  expansion  of  the  steam  occurred  in  a  single 
cylinder  having  no  clearance.  The  line  AB  represents  the  admis- 
sion line,  which  is  very  short  compared  with  the  total  length 
of  the  stroke,  which  is  represented  by  the  line  GF,  and  which 


100- 


80- 


4-0 


20- 


P  a.  \Z( 
t    a  34 


0 


=   19  Lb.t    t»  225* 


P  =   1.6    Lb.,  t  =  I  ISC 


FIG.  146. 

also  represents  the  volume  of  the  steam  after  expansion.  The 
line  GF  therefore  also  represents  the  volume  of  the  cylinder.  The 
short  admission  line  is  necessary  if  the  steam  is  to  be  expanded 
through  the  entire  range  of  pressure  in  one  cylinder.  It  will 
be  observed  that  if  the  entire  expansion  occurred  in  a  single 
cylinder,  this  cylinder  would  have  to  be  large  enough  to  accommo- 
date the  entire  volume  of  steam  GF,  and  would  have  to  be  strong 
enough  to  withstand  the  full  pressure  of  120  Ib.  per  sq.  in.  The 
objection  to  this  would  be  that  the  cost  of  such  a  cylinder  would 
be  excessive;  there  would  be  a  waste  of  power  in  overcoming 
friction;  and  the  large  size  of  the  cylinder  would  expose  so  much 
surface  to  the  cooling  action  of  the  exhaust  steam  that  condensa- 
tion would  be  excessive. 

Now,  suppose  that  a  line  JD  be  drawn  across  the  diagram  at 


228  STEAM  ENGINES 

such  a  height  that  the  area  of  the  diagram  will  be  divided  into 
two  approximately  equal  parts.  If  the  two  parts  into  which  the 
expansion  of  the  steam  is  divided  are  performed  in  separate 
cylinders,  the  first  one  will  have  a  volume  JC  and  would  be  built 
to  withstand  the  full  steam  pressure  of  120  Ib.  per  sq.  in.  This 
cylinder  would  admit  a  volume  of  steam  AB  and  would  expand 
it  to  the  volume  JC.  The  expansion  would  not  be  carried  further 
than  the  point  C  because  it  is  desirable  to  have  enough  pressure 
in  the  cylinder  at  release  to  force  the  steam  out  of  the  cylinder 
rapidly,  and  also  because  the  extra  amount  of  work  obtained 
by  complete  expansion  to  the  point  D  would  not  be  enough  to 
balance  the  work  lost  in  friction  while  the  piston  was  moving 
through  this  part  of  the  stroke.  The  exhaust  from  the  first 
cylinder  would  form  the  supply  for  the  second  cylinder.  This 
cylinder  would  have  a  volume  GF,  the  same  as  would  a  cylinder 
designed  for  the  entire  expansion,  but,  as  the  supply  of  steam 
for  the  second  cylinder  has  a  pressure  of  only  19  Ib.  per  sq.  in. 
it  would  not  have  to  be  so  strong  as  a  single  cylinder  designed 
for  120  Ib.  pressure,  hence  would  be  cheaper  to  construct. 

The  second  cylinder  would  admit  the  volume  of  steam  JD  at  a 
pressure  of  19  Ib.  per  sq.  in.  and  would  expand  it  to  the  volume 
GF,  when  its  pressure  would  be  1.6  Ib.  per  sq.  in.  If  the  total 
expansion  occurred  in  a  single  cylinder,  this  cylinder  would  be 
subjected  to  the  full  range  of  temperature,  223°,  and,  since  its 
wall  surface  would  be  large,  condensation  would  be  excessive. 
By  dividing  the  expansion  into  two  parts,  each  cylinder  experi- 
ences a  range  of  temperature  of  only  about  112°,  that  is,  the 
range  of  temperature  has  been  cut  in  half  and  the  cylinder  sur- 
face has  not  been  doubled,  hence  the  condensation  and  reevapo- 
ration  in  the  two  cylinders  would  be  less  than  in  a  single  cylinder 
subjected  to  the  entire  range  of  temperature.  This  decreases 
materially  the  large  loss  of  heat  that  would  otherwise  occur 
through  condensation  and  reevaporation ;  but,  on  the  other 
hand,  the  engine  would  be  more  complicated  and  therefore  more 
expensive,  and  the  friction  loss  would  be  increased  by  the  greater 
number  of  moving  parts. 

Compound  Engines. — Compound  engines  are  divided  into 
two  classes,  based  upon  the  arrangement  of  cylinders.  These 
are  called  tandem-compound,  in  which  one  cylinder  is  placed 
behind  the  other,  and  cross-compound,  in  which  the  cylinders 
are  placed  parallel  with  each  other. 


COMPOUND  ENGINES  229 

The  tandem  engine,  as  illustrated  in  Fig.  147,  has  only  one 
piston  rod,  connecting  rod,  and  crank.  The  piston  rod  extends 
from  one  cylinder  through*;  the  other  and  has  both  pistons 


FIG.   147. 


attached  to  it.  The  exhaust  pipe  from  the  high  pressure  cylin- 
der passes  directly  to  the  low  pressure  cylinder,  and,  as  this  pipe 
is  short,  it  has  but  little  storage  capacity;  therefore  it  may  be 
considered  that  the  high  pressure  cylinder  exhausts  directly  into 


FIG.   148. 


the  low  pressure  cylinder.  The  tandem-compound  engine  is 
simple  in  construction,  but  the  parts  must  be  made  large  in  order 
to  carry  the  heavy  stresses. 

The  cross-compound  engine,  illustrated  in  Fig.  148,  has  two 


230 


STEAM  ENGINES 


pistons,  piston  rods,  connecting  rods,  and  cranks,  hence  it  is 
similar  to  two  simple  engines  placed  parallel  with  each  other  and 
connected  to  the  same  shaft.  The  cranks  are  usually  placed 
90°  apart,  which  gives  a  more  uniform  turning  effort  on  the  shaft. 
Since  each  side  of  the  engine  transmits  only  one-half  of  the  power, 
the  parts  of  the  engine  are  made  smaller,  but  the  larger  number 
of  parts  makes  this  type  of  engine  more  expensive  than  the  tan- 
dem-compound. The  exhaust  pipe  from  the  high-pressure 
cylinder  extends  across  to  the  low-pressure  cylinder  and  contains 
a  receiver  or  vessel  in  which  steam  may  be  stored.  This  is  made 
necessary  by  the  cranks  being  placed  90°  apart,  as  explained  in  a 
later  paragraph. 

The  action  of  the  steam  in  the  two  classes  of  engines  mentioned 


L.INE 


L.  P. 


FIG.  149. 

above  is  quite  different.  In  the  tandem  engine  the  pistons 
have  the  same  length  of  stroke,  and  move  in  unison  with  each 
other,  beginning  a  stroke  at  the  same  time  and  ending  it  at  the 
same  time.  For  this  reason  the  steam  exhausted  from  the  high- 
pressure  cylinder  may  be  passed  directly  into  the  low-pressure 
cylinder  without  having  any  valves  on  the  latter  cylinder,  and 
without  any  storage  space  or  receiver  between  the  cylinders. 
In  this  case  the  valves  and  governor  on  the  high-pressure  cylin- 
der control  the  action  of  the  steam  and  the  amount  of  work 
performed  in  both  cylinders. 

Cross-Compound  Engines.— The  action  of  the  steam  in  both 
cylinders  of  a  cross-compound  engine  with  cranks  set  180°  apart 
and  without  valves  on  the  low-pressure  cylinder  is  very  similar 


COMPOUND  ENGINES  231 

to  that  in  a  tandem-compound  engine,  and  may  be  studied  best 
by  considering  the  indicator  diagrams  shown  in  Fig.  149.  This 
illustration  shows  the  diagram  from  the  high-pressure  cylinder, 
marked  H.P.  and  that  from  the  low-pressure  cylinder,  marked 
L.P.,  placed  in  their  correct  relative  positions,  that  is,  so  that  the 
exhaust  stroke  for  the  high-pressure  cylinder  is  the  admission 
stroke  for  the  low-pressure  cylinder.  These  diagrams  do  not, 
however,  show  correctly  the  division  of  work  between  the  cylin- 
ders, because,  being  drawn  to  the  same  scale  of  pressure  and 
stroke,  they  do  not  take  into  account  the  different  diameters  of 
the  cylinders. 

After  the  supply  of  steam  is  cut  off  from  the  high-pressure 
cylinder,  the  steam  expands  in  this  cylinder  until  released. 
During  exhaust  from  the  high-pressure  cylinder,  the  steam 
flows  directly  into  the  low-pressure  cylinder.  Since  the  diameter 
of  the  low-pressure  cylinder  is  larger  than  that  of  the  high-pres- 
sure cylinder  and  both  pistons  move  at  the  same  speed,  the 
volume  displaced  in  the  low-pressure  cylinder  is  greater  than 
that  displaced  in  the  high-pressure  cylinder.  The  result  of  this 
is  that  each  cubic  foot  of  exhaust  steam  pushed  out  of  the  high- 
pressure  cylinder  flows  into  a  larger  volume  than  one  cubic  foot 
in  the  low-pressure,  and  its  pressure  therefore  falls.  This  is 
why  the  exhaust  from  the  high-pressure  cylinder  and  the  admis- 
sion to  the  low-pressure  cylinder  show  a  continually  falling 
pressure.  When  the  point  of  compression  in  the  high-pressure 
cylinder  is  reached,  the  supply  of  steam  for  the  low-pressure 
cylinder  is  stopped  and  the  steam  then  in  the  low-pressure  cyl- 
inder expands  with  a  rapidly  falling  pressure,  since  no  new 
steam  is  being  supplied. 

It  will  be  observed  from  Fig.  149  that  the  range  in  temperature 
in  the  high-pressure  cylinder  is  that  represented  by  the  change 
in  pressure  from  A  to  C,  which  is  greater  than  it  would  have  been 
if  there  was  less  drop  in  pressure  during  exhaust.  Also  the  range 
in  temperature  in  the  low-pressure  cylinder  is  that  due  to  the 
difference  in  pressure  between  E  and  the  exhaust  pressure  from 
the  low-pressure  cylinder.  Since  the  pressure  at  E  is  greater 
than  at  C,  the  range  in  temperature  is  greater  in  both  cylinders 
than  would  be  indicated  by  a  division  of  the  work  into  two  equal 
parts. 

The  above  analysis  of  the  action  of  steam  in  the  cylinders  of  a 
cross-compound  engine  applies  only  to  those  engines  which  have 


232 


STEAM  ENGINES 


no  valves  on  the  low-pressure  cylinder  or  to  those  engines  which 
have  only  one  valve  for  both  cylinders  and  this  valve  so  arranged 
that  cut-off  in  the  low-pressure  cylinder  occurs  at  the  same  time 
as  compression  in  the  high-pressure  cylinder.  This  type  of 
engine  is  not  used  to  a  large  extent  and  is  made  only  in  compara- 
tively small  sizes.  A  more  common  arrangement,  either  in 
tandem-compound  engines  or  in  cross-compound  engines  with 
cranks  placed  90°  apart,  is  to  have  separate  valves  on  each 
cylinder  which  may  be  adjusted  independently  of  each  other. 
Engines  of  this  kind  must  necessarily  be  supplied  with  a  receiver 
or  storage  space  in  which  the  exhaust  steam  from  the  high-pres- 
sure cylinder  may  be  stored  if  cut-off  in  the  low-pressure  cylinder 


FIG.  150. 

does  not  occur  at  the  same  time  as  compression  in  the  high- 
pressure  cylinder.  If  the  cylinders  are  placed  near  each  other  so 
that  the  connecting  passages  are  short,  the  receiver  is  usually 
in  the  form  of  a  separate  vessel  connected  in  the  passage  between 
the  two  cylinders;  but  when  the  cylinders  are  some  distance 
apart,  the  passage  connecting  the  two  cylinders  has  enough 
volume  to  act  as  a  receiver,  and  no  separate  vessel  is  necessary. 
Tandem-Compound  Engines. — The  presence  of  a  receiver 
modifies  somewhat  the  action  of  the  steam  in  the  cylinders  from 
that  described  above  and  illustrated  in  Fig.  149.  For  a  tandem- 
compound  engine  in  which  the  connecting  passage  acts  as  a 
receiver,  or  for  a  cross-compound  engine  with  cranks  180°  apart 
and  supplied  with  a  receiver,  the  action  of  the  steam  in  the  cylin- 
ders may  be  shown  by  the  diagrams  in  Fig.  150,  which  are  similar 


COMPOUND  ENGINES  233 

to  those  shown  in  Fig.  149  except  that  cut-off  in  the  low-pressure 
cylinder  does  not  occur  at  i^he  same  time  that  compression  occurs 
in  the  high-pressure  cylinder. 

In  this  case  it  will  be  observed  from  Fig.  150  that  cut-off  in 
the  low-pressure  cylinder  occurs  a  little  after  half  stroke  and 
considerably  before  compression  (marked  D)  occurs  in  the  high- 
pressure  cylinder.  When  cut-off  occurs  in  the  low-pressure 
cylinder,  the  steam  then  in  that  cylinder  expands  in  the  usual 
manner.  The  high-pressure  cylinder,  however,  has  not  finished 
exhausting  at  this  time;  hence  the  remainder  of  the  exhaust 
from  the  high-pressure  cylinder  is  stored  in  the  receiver.  Since 
no  steam  is  being  drawn  from  the  receiver  at  this  time,  the 
pressure  in  it,  which  is  also  the  exhaust  pressure  of  the  high- 
pressure  cylinder,  increases  as  shown  by  the  line  CD  in  Fig. 
150.  At  D  compression  occurs  in  the  high-pressure  cylinder  and 
the  exhaust  valve  closes  communication  with  the  receiver. 

The  point  of  cut-off  in  the  low-pressure  cylinder  controls  the 
increase  of  pressure  in  the  receiver,  from  C  to  D,  the  increase  of 
pressure  being  greater  with  an  early  cut-off  and  smaller  with  a 
later  cut-off.  The  cut-off  in  the  low-pressure  cylinder  must  be 
so  timed  that  the  pressure  in  the  receiver  will  be  the  same  at  D 
as  at  E,  the  point  where  the  exhaust  valve  on  the  high-pressure 
cylinder  opens.  If  the  pressure  at  D  is  not  so  high  as  at  E, 
the  pressure  at  the  end  of  expansion  in  the  high-pressure  cylinder 
will  be  greater  than  that  in  the  receiver  and  there  will  be  a  drop 
of  pressure  the  next  time  the  exhaust  valve  on  the  high-pressure 
cylinder  opens.  This  would  cause  a  waste  of  pressure  and  a  loss 
of  work,  which  is  to  be  avoided  if  possible. 

Cross-Compound  with  Receiver. — The  action  of  steam  in  the 
cylinders  of  a  cross-compound  engine  with  cranks  set  90°  apart 
presents  another  interesting  case.  An  engine  of  this  kind  must 
necessarily  be  supplied  with  a  receiver  because  one  piston  is  at 
mid-stroke  when  the  other  is  at  the  end  of  its  stroke;  hence, 
exhaust  from  the  high-pressure  cylinder  progresses  for  one-half 
of  a  stroke  when  no  steam  is  being  admitted  to  the  low-pressure 
cylinder,  and  it  is  necessary  to  have  a  receiver  in  which  to  store 
this  steam. 

The  diagrams  from  the  high-  and  low-pressure  cylinders  of  an 
engine  of  this  type  are  shown  in  Fig.  151.  These  diagrams  are 
not  drawn  in  the  usual  manner,  but  instead,  the  low-pressure 
diagram  is  displaced  one-half  stroke  from  the  high-pressure 


234 


STEAM  ENGINES 


.diagram  in  order  to  show  the  relative  pressures  in  the  cylinders 
at  any  instant. 

It  will  be  observed  from  Fig.  151  that  the  exhaust  pressure  in 
the  high-pressure  cylinder  increases  gradually  from  the  beginning 
to  the  middle  of  the  exhaust  stroke.  The  reason  for  this  is  that 
during  this  part  of  the  stroke  the  high-pressure  cylinder  is  ex- 
hausting into  the  receiver  and  the  low-pressure  cylinder  is  not 
taking  any  steam  from  it;  hence  the  exhaust  pressure  in  the 
high-pressure  cylinder,  which  is  also  the  receiver  pressure,  in- 
creases. When  the  high-pressure  piston  reaches  mid-stroke, 
the  low-pressure  cylinder  begins  to  admit  steam,  since  the  cranks 
are  90°  apart,  and  the  receiver  pressure  is  reduced.  Thus,  the 


FIG.  151. 

high-pressure  exhaust  line  rises  from  beginning  to  mid-stroke 
and  falls  from  mid-stroke  to  the  point  of  compression. 

The  admission  line  for  the  low-pressure  cylinder  follows  the 
shape  of  the  last  half  of  the  exhaust  line  of  the  high-pressure 
cylinder;  hence  it  shows  a  decreasing  pressure.  In  the  low- 
pressure  diagram  shown  in  Fig.  151  cut-off  occurs  at  or  before 
mid-stroke,  or  before  the  high-pressure  piston  has  completed 
its  stroke.  If  cut-off  in  the  low-pressure  cylinder  occurs  after 
half  stroke  the  high-pressure  piston  will  have  started  on  its  return 
stroke  and  exhaust  will  have  commenced  from  the  other  end  of 
the  cylinder;  hence  the  pressure  in  the  receiver  will  again  begin 
to  increase  and  this  will  produce  a  corresponding  increase  in  the 
admission  pressure  for  the  low-pressure  cylinder.  The  effect 
of  the  second  admission  of  steam  into  the  receiver  before  the  low- 


COMPOUND  ENGINES  235 

pressure  cut-off  is  illustrated  in  Fig.  152,  where  the  admission 
pressure  for  the  low-pressure  cylinder  is  shown  decreasing  up  to 
mid-stroke  and  increasing  fp&m  mid-stroke  to  the  point  of  cut- 
off. This  second  increase  in  pressure  is  called  "second  admis- 
sion, "  and  is  to  be  found  only  when  cut-off  in  the  low-pressure 
cylinder  occurs  after  mid-stroke. 

One  of  the  advantages  of  the  cross-compound  engine  with 
cranks  90°  apart  is  illustrated  by  Fig.  151  which  shows  that  the 
range  of  temperature  in  it  is  less  than  in  the  cylinders  of  a  cross- 
compound  with  cranks  set  180°  apart  (Fig.  149),  or  a  tandem- 
compound  (Fig.  150),  because  the  exhaust  from  the  high-pressure 
cylinder  shows  a  more  uniform  pressure.  The  variations  in 
pressure  illustrated  in  Figs.  149,  150,  151,  and  152  will  not  show 
to  such  a  marked  degree  on  the  actual  indicator  diagrams  because 


FIG.   152. 

the  high-pressure  diagram  is  drawn  with  a  stiff  indicator  spring. 
The  variations  in  pressure  in  the  low-pressure  admission  may  be 
detected  easily  on  the  actual  diagram,  however,  because  this 
diagram  is  drawn  with  a  weak  spring. 

Power  of  a  Compound  Engine. — The  power  developed  by  any 
steam  engine,  whether  simple  or  compound,  depends  upon  the 
number  of  times  the  steam  is  expanded,  that  is,  upon  its  ratio 
of  expansion.  It  evidently  does  not  matter,  then,  as  far  as  the 
power  of  a  compound  engine  is  concerned,  whether  the  total 
expansion  of  the  steam  occurs  in  one  cylinder  or  is  divided 
between  two  cylinders,  provided  only  that  the  steam  is  expanded 
the  same  number  of  times  in  each  case. 

In  a  compound  engine,  the  total  expansion  is  divided  between 
two  cylinders  for  the  purpose  of  reducing  cylinder  condensation, 
and  not  for  the  purpose  of  increasing  the  power  of  the  engine. 
The  total  power  developed  in  both  cylinders  of  a  compound 
engine  could  be  developed  in  the  low-pressure  cylinder  alone 


236  STEAM  ENGINES 

by  having  cut-off  in  the  low-pressure  cylinder  occur  early  enough 
to  secure  as  many  expansions  of  the  steam  in  this  cylinder  as  was 
secured  in  both  the  high-  and  low-pressure  cylinders.  For  exam- 
ple, if  cut-off  in  the  high-pressure  cylinder  occurs  at  one-quarter 
stroke  the  steam  will  expand  approximately  four  times  in  this 
cylinder.  If  the  volume  of  the  low-pressure  cylinder  is  three 
times  that  of  the  high-pressure  cylinder  then  the  total  expansion 
of  the  steam  will  be 

4  X  3  =  12 

This  number  of  expansions  could  have  been  secured  in  the  low- 
pressure  cylinder  alone  by  admitting  the  steam  directly  to  that 
cylinder  and  having  cut-off  occur  at  approximately  ^{2  stroke. 

In  any  case  the  approximate  ratio  of  expansion  in  a  multiple 
expansion  engine  may  be  found  by  multiplying  the  ratio  of  expan- 
sion in  the  high-pressure  cylinder  by  the  ratio  of  the  volume  of 
the  low-pressure  to  the  high-pressure  cylinder,  or  by  the  ratio 
of  the  square  of  their  diameters.  In  order  to  find  the  ratio  of 
expansion  more  accurately,  the  clearance  volumes  would  have 
to  be  considered,  but  this  is  not  necessary  for  ordinary  purposes, 
as  the  ratio  of  expansion  changes  with  the  cut-off  which,  in  turn, 
varies  with  the  load. 

From  the  above  discussion  it  will  be  seen  that  the  total  power 
developed  by  a  compound  engine  depends  upon  the  ratio  of 
the  cylinder  volumes  and  upon  the  point  of  cut-off  in  the  high- 
pressure  cylinder.  For  any  given  engine  the  ratio  of  the  cylinder 
volumes  is  a  fixed  quantity,  therefore  we  may  say  as  a  general 
proposition  that  the  power  developed  by  a  compound  engine 
depends  only  upon  the  point  of  cut-off  in  the  high-pressure  cylinder. 

The  point  of  cut-off  in  the  low-pressure  cylinder  has  no  effect 
whatever  upon  the  total  amount  of  work  done  by  a  compound 
engine.  The  point  of  cut-off  in  the  low-pressure  cylinder  does, 
however,  control  the  distribution  of  work  between  the  two  cylinders. 
It  does  this  by  affecting  the  exhaust  pressure  of  the  high-pressure 
cylinder.  If  cut-off  in  the  low-pressure  cylinder  occurs  early  in 
the  stroke,  the  exhaust  pressure  of  the  high-pressure  cylinder 
will  be  high  and  the  work  performed  in  this  cylinder  will  be  a 
smaller  portion  of  the  total  work  and  the  work  performed  in  the 
low-pressure  cylinder  will  be  a  larger  portion  of  the  total  work. 
On  the  other  hand,  if  cut-off  in  the  low-pressure  cylinder  occurs 
late,  the  exhaust  pressure  of  the  high-pressure  cylinder  will  be 


COMPOUND  ENGINES 


237 


lower  and  a  larger  proportion  of  the  total  work  will  be  performed 
in  the  high-pressure  cylinder. 

The  low-pressure  cut-of^; should  be  adjusted  so  as  to  secure 
an  approximately  equal  division  of  work  between  the  high-  and 
low-pressure  cylinders,  and,  also,  so  there  will  be  a  small  drop  in 
pressure  at  the  end  of  expansion  in  the  high-pressure  cylinder 
when  the  engine  is  working  under  load.  The  object  in  having 
a  small  drop  of  pressure  at  the  end  of  expansion  is  that  there  will 
be  no  gain  in  carrying  the  expansion  completely  down  to  exhaust 
pressure  and,  moreover,  a  little  drop  in  pressure  into  the  receiver 
is  needed  tp  secure  a  quick  flow  of  steam  out  of  the  high-pressure 


f  Steam:  140*  Gauge 

Normal  Conditions:    4  Vacuum:  25" to  27"  Hg. 
/Speed:  120  rp.m. 


Scale:  80^=1 
Cylinder-.  14"  x  36" 


Vacuum  Line 


H.R    DIAGRAMS 


A 

Scale-.  20*=  I" 
Cylinder  28"  x  36" 


L.R  DIAGRAMS 

FIG.  152a. 

cylinder.  However,  when  the  engine  is  working  under  no  load 
or  only  a  small  load  there  should  be  no  drop  of  pressure  into  the 
receiver,  but  instead,  the  receiver  pressure  should  be  higher  than 
the  pressure  at  the  end  of  expansion  in  the  high-pressure  cylinder. 
The  importance  of  this  should  not  be  overlooked,  because,  if  the 
receiver  pressure  becomes  too  low,  a  condition  may  be  produced 
under  which  the  engine  will  run  away. 

Such  a  condition  as  this  is  illustrated  in  Figs.  152a  and  1526. 
These  illustrations  show  the  indicator  diagrams  from  a  cross- 
compound  engine  in  which  the  condition  mentioned  above 
existed.  Fig.  152a  shows  the  indicator  diagrams  from  the  high- 
pressure  cylinder  at  A  and  those  from  the  low-pressure  cylinder 
at  A'j  both  being  taken  while  the  engine  was  running  under  no 


238 


STEAM  ENGINES 


load.  It  will  be  noted  by  examining  these  diagrams  that  the 
receiver  pressure  is  too  low,  as  indicated  by  the  exhaust  line  of 
the  high-pressure  diagrams  and  also  by  the  admission  lines  of  the 
low-pressure  diagrams.  The  exhaust  pressure  of  the  high-pres- 
sure cylinder  is  so  low  that  it  is  impossible  for  negative  work  to  be 
done  in  the  high-pressure  cylinder,  and,  even  though  the  governor 
is  causing  cut-off  at  the  earliest  possible  point,  the  expansion 
of  steam  in  the  high-pressure  cylinder  is  doing  more  work  than 
needed  to  carry  the  friction  load  at  normal  speed.  Consequently 
the  speed  increases.  When  the  speed  had  reached  140  R.p.m.  the 


|5teanrv.  \  40     Gauge 

Normal  Conditions:    <  Vacuum;  25"to  27"  Hg 
I  Speed--  120  rp.m. 


B 

Scale-.  80*=l" 
Cylinder-  I4"x3e" 


Atmospheric  Line 


Vacuum  Line 


H.P.  DIAGRAM 


Atmospheric  Line 


B7 

Scale:20*=V" 
Cylinder:  28"  x  36 


Vacuum  \_in* 


L.P.  DIAGRAM 
FIG.  1526. 

engine  was  stopped  by  closing  the  throttle,  but  if  this  had  not 
been  done  the  speed  would  have  continued  to  increase  and  the 
engine  would  have  run  away.  If  the  cut-off  in  the  low  pressure 
had  occurred  earlier  in  the  stroke,  the  receiver  pressure,  which 
is  the  exhaust  pressure  of  the  high-pressure  cylinder,  would  have 
been  higher  and  negative  work  would  have  been  done  in  the  high- 
pressure  cylinder.  This  would  have  put  sufficient  load  on  the 
high-pressure  cylinder  to  prevent  the  speed  from  increasing  above 
its  normal  value. 

Fig.  1526  shows  diagrams  taken  from  the  engine  when  running 
under  no  load  but  with  the  cut-off  in  the  low-pressure  cylinder 
adjusted  to  occur  earlier.  It  will  be  observed  that  the  receiver 


COMPOUND  ENGINES  239 

pressure  is  now  7J^  to  8  Ibs.  above  atmospheric  pressure  and  the 
expansion  in  the  high-pressure  cylinder  carries  the  pressure  below 
receiver  pressure  so  that  negative  work  (indicated  by  the  cross- 
hatched  lines)  is  done  by  the  high-pressure  piston  during  its 
exhaust  stroke.  This  negative  work  is  sufficient  to  hold  the 
speed  down  almost  to  normal. 

Advantages  and  Disadvantages. — The  principal  advantage 
derived  from  compounding  is  the  reduction  of  losses  resulting 
from  cylinder  condensation  and  reevaporation.  With  a  simple 
engine  these  losses  increase  with  high-steam  pressures  and  with  a 
large  number  of  expansions  of  the  steam.  Hence,  a  simple 
engine  is  not  well  adapted  to  the  use  of  high  pressures  nor  for 
an  early  cut-off,  both  of  which  are  necessary  for  the  economical 
use  of  the  steam.  It  will  now  be  understood  why  a  compound 
engine  is  much  better  adapted  for  the  use  of  high-pressure  steam 
and  for  expanding  the  steam  a  large  number  of  times,  hence  the 
general  use  of  this  type  of  engine  for  producing  large  amounts  of 
power,  when  efficiency  is  a  very  important  factor. 

Most  of  the  disadvantages  of  the  compound  engine  are  of  a 
mechanical  nature  and  arise  from  the  greater  complication  of  this 
type.  The  greater  number  of  parts  make  them  more  expensive  in 
first  cost  and  also  make  them  more  expensive  to  maintain  than  a 
simple  engine,  on  account  of  more  repairs  being  necessary.  The 
greater  number  of  moving  parts  also  adds  to  the  cost  of  lubrica- 
tion and  increases  the  loss  of  power  in  friction.  In  these  respects 
triple  expansion  and  quadruple  expansion  engines  are  at  even 
greater  disadvantage  than  compound  engines,  with  the  result 
that  quadruple  expansion  engines  have  dropped  out  of  use  for 
stationary  purposes  and  the  use  of  triple  expansion  engines  is 
confined  almost  entirely  to  large  pumping  plants. 


CHAPTER  XVIII 
CONDENSING  APPARATUS 

Purpose  of  the  Condenser. — When  an  engine  exhausts  into  the 
atmosphere,  the  exhaust  stroke  of  the  piston  is  made  against  the 
atmospheric  pressure,  which  acts  upon  the  entire  face  of 
the  piston.  This  pressure  acts  in  a  direction  opposite  to  that  in 
which  the  piston  is  moving  and  tends  to  retard  its  motion.  The 
piston  must  overcome  not  only  the  atmospheric  pressure  but 
also  the  friction  of  the  exhaust  steam  in  passing  through  the 
ports  and  exhaust  pipe  on  its  way  from  the  cylinder  to  the 
atmosphere.  The  atmospheric  pressure  (14.7  Ib.  per  sq.  in.) 
added  to  the  friction  of  the  exhaust  passages  makes  a  total  pres- 
sure of  between  15  and  20  Ibs.  per  sq.  in.  which  the  piston  must 
move  against.  When  it  is  considered  that  this  back  pressure 
acts  upon  the  piston  during  almost  the  entire  exhaust  stroke, 
and  that  the  piston  must  do  work  in  moving  against  this  pressure, 
it  will  be  realized  that  the  engine  could  do  considerably  more 
useful  work  if  this  back  pressure  were  removed. 

Removing  or  reducing  the  back  pressure  on  an  engine  increases 
its  mean  effective  pressure.  When  it  is  remembered  that  the 
mean  effective  pressure  of  an  engine  is  directly  proportional  to  its 
indicated  horsepower,  it  will  be  seen  that  anything  which  in- 
creases the  mean  effective  pressure  will  also  increase  the  indicated 
horsepower  developed  by  the  engine.  In  order  to  gain  some  idea 
of  the  increase  in  horsepower  by  lowering  the  back  pressure 
consider  an  engine  taking  steam  at  an  absolute  pressure  of  100 
Ib.  per  sq.  in.,  cutting  off  at  Y±  stroke,  and  exhausting  into  the 
atmosphere  against  a  back  pressure  of  16  Ib.  per  sq.  in.  The 
theoretical  mean  effective  pressure  under  these  conditions  will 
be  43.7  Ib.  per  sq.  in.  If  the  back  pressure  was  reduced  to  1.7 
Ib.  per  sq.  in.  (26  in.  vacuum)  the  mean  effective  pressure  would 
be  increased  to 

43.7  +  (16  -  1.7)  =  58  Ib.  per  sq.  in. 

which  would  result  iji  an  increase  of  power  of 

240 


CONDENSING  APPARATUS  241 

100  ^rf^7  =  32.8  per  cent. 

S 

The  actual  gain  would  be  somewhat  less  than  this  depending 
upon  the  type  of  engine  and  the  conditions  of  operation,  but 
in  any  case  it  would  be  considerable. 

The  back  pressure  on  an  engine  is  reduced  by  leading  the 
exhaust  steam  into  a  closed  vessel  and  condensing  it  into  water, 
instead  of  permitting  the  engine  to  exhaust  directly  into  the 
atmosphere.  Such  a  closed  vessel  is  called  a  condenser.  As  the 
exhaust  steam  enters  the  condenser,  it  either  meets  a  spray  of 
cold  water  or  comes  in  contact  with  tubes  through  which  cold 
water  is  flowing.  In  either  case,  the  water  extracts  heat  from  the 
exhaust  steam  and  condenses  it  into  water.  Since  the  water 
occupies  only  about  MTOO  of  the  space  occupied  by  the  exhaust 
steam,  the  pressure  in  the  condenser  is  reduced  by  the  condensa- 
tion of  the  steam.  In  order  to  maintain  the  low  pressure  in 
the  condenser  it  is  necessary  to  condense  the  exhaust  steam  as 
fast  as  it  enters  and  to  constantly  remove  the  water  and  any  air 
which  may  come  in  with  the  exhaust  steam. 

The  purpose  of  installing  a  condenser  may  be  either  to  increase 
the  efficiency  of  the  engine  or  to  increase  the  power  of  the  engine. 
A  condensing  engine  will  be  more  efficient  than  a  noncondensing 
one  for  the  reason  that  cut-off  in  the  condensing  engine  may  be 
shorter  than  in  the  noncondensing  engine  when  the  same  amount 
of  power  is  developed  in  both,  on  accountof  the  great  number  of 
times  the  steam  is  expanded  in  the  condensing  engine.  For 
example,  an  engine  running  noncondensing  may  cut  off  at  }/± 
stroke  and  develop  a  certain  amount  of  power.  The  same 
engine  connected  to  a  condenser  may  cut  off  at  %  stroke  and 
develop  the  same  amount  of  power.  Since  the  amount  of  steam 
used  by  the  engine  is  in  proportion  to  the  cut-off,  the  engine  will 
use  %  — .  J£  =  J^  less  steam  when  running  condensing  than 
when  running  noncondensing.  The  amount  of  steam  which  can 
be  produced  from  a  pound  of  coal  is  ordinarily  from  7  to  10 
pounds,  but  the  amount  of  power  obtained  from  the  steam 
depends  upon  how  the  steam  is  utilized.  Since,  by  running  an 
engine  condensing  rather  than  noncondensing  the  steam  is 
utilized  more  efficiently,  power  plant  engines  are  almost  invari- 
ably run  condensing  unless  the  exhaust  steam  is  used  for  heating. 

A  Corliss  engine  running  noncondensing  will  use  from  25  to  30 
pounds  of  steam  per  indicated  horsepower  per  hour  but  if  run 

24 


242  STEAM  ENGINES 

condensing,  it  will  use  only  about  20  pounds.  For  compound 
engines,  the  amount  used  will  be  about  25  pounds  noncondensing 
and  about  15  pounds  condensing.  A  triple  expansion  engine 
running  condensing  will  produce  an  indicated  horsepower  from 
as  little  as  10  pounds  of  steam. 

While  a  condensing  engine  will  require  from  20  to  30  per  cent, 
less  steam  than  a  noncondensing  one,  this  apparent  decrease  in 
steam  consumption  does  not  represent  a  net  gain.  The  steam 
used  by  the  condenser  pumps  must  be  added  to  that  consumed 
by  the  engine  unless  the  exhaust  from  the  pumps  is  used  for 
heating  the  feed  water  in  which  case  only  the  difference  between 
the  heat  entering  and  leaving  the  pumps  should  be  charged  to 
the  engine. 

Condensation  of  Steam. — The  condensation  of  steam  is  just 
the  reverse  of  the  process  by  which  steam  is  formed,  and  the 
amounts  of  heat  involved  are  the  same ;  the  only  difference  being 
that  heat  must  be  added  to  water  to  change  it  into  steam  and 
that  heat  must  be  taken  away  from  the  steam  to  condense  it  into 
water.  Moreover,  for  the  same  conditions  of  pressure,  quality, 
and  weight  of  steam  exactly  the  same  amount  of  heat  must  be 
transferred  in  either  case,  being  transferred  into  the  steam  in  one 
case  and  out  of  it  in  the  other. 

A  pound  of  steam  at  any  pressure  contains  a  definite  amount  of 
latent  heat  of  evaporation,  as  may  be  seen  by  reference  to  the 
steam  table  in  Chapter  5.  If  this  amount  of  heat  is  taken  out  of 
the  steam,  a  pound  of  it  condenses  into  water  and  the  water  will 
have  the  same  temperature  as  the  steam  from  which  it  was  con- 
densed. If  only  one-half  of  the  latent  heat  in  a  pound  of  steam 
is  extracted,  then  only  one-half  of  a  pound  of  steam  will  be  con- 
densed and  the  resulting  water  will  be  at  the  same  temperature 
as  the  steam.  The  same  is  true  for  any  amount  of  heat  taken 
from  the  steam.  The  weight  of  steam  condensed  will  be  the 
number  of  heat  units  extracted  divided  by  the  latent  heat  of  one 
pound  of  steam  at  the  pressure  of  condensation.  If  the  exhaust 
steam  is  wet,  that  is,  contains  moisture  suspended  in  it,  this 
moisture  contains  no  latent  heat,  therefore  only  the  latent  heat 
actually  contained  in  the  steam  must  be  extracted  in  order  to 
condense  it.  While  steam  may  not  be  entirely  dry  at  the  end 
of  expansion,  the  drop  in  pressure  at  release  usually  completes 
the  drying,  so  that  in  calculations  relating  to  condensers  it  is 
usually  assumed  that  the  exhaust  steam  is  dry. 


CONDENSING  APPARATUS  243 

In  order  to  condense  steam  it  is  necessary  to  bring  it  into  con- 
tact with  something  which  Jias  a  lower  temperature  because  heat 
will  only  pass  into  a  substance  at  lower  temperature.  For  this 
reason  the  condensing  water  used  in  a  condenser  must  have  a 
lower  temperature  than  the  exhaust  steam  that  is  to  be  condensed. 
When  the  condensing  water  absorbs  heat  from  the  exhaust  steam, 
the  steam  is  condensed  and  the  temperature  of  the  condensing 
water  is  increased.  It  is  evident  that  the  steam  cannot  be  con- 
densed unless  its  temperature  is  higher  than  the  final  temperature 
of  the  condensing  water. 

By  continually  condensing  the  steam  in  the  condenser  a  low 
pressure  is  maintained  in  it.  The  steam  is  expanded  in  the 
cylinder  almost  to  this  pressure,  and  when  the  exhaust  valve 
opens,  the  steam  pressure  drops  to  the  same  pressure  as  that  in 
the  condenser.  At  the  same  time,  its  temperature  drops  to  that 
shown  by  the  steam  table  to  correspond  to  its  pressure.  For 
example,  suppose  the  absolute  pressure  in  the  condenser  is 
maintained  at  2  Ib.  per  sq.  in.  then  the  exhaust  steam  entering 
it  will  have  this  pressure  and  it  will  have  a  temperature  of  126.15° 
F.  as  will  be  seen  by  referring  to  the  steam  table  in  Chapter  5. 
The  exhaust  steam  at  this  pressure  and  temperature  has  a  latent 
heat  of  1021  B.t.u.  per  pound,  and  in  condensing  it  gives  up  this 
heat  to  the  condensing  water.  The  condensate,  or  water  result- 
ing from  the  condensation  of  the  steam,  will  also  have  a  tempera- 
ture of  126.15°F. 

Measuring  Vacuum. — Strictly  speaking,  a  vacuum  means  a 
space  in  which  there  is  no  pressure,  or  in  which  the  absolute 
pressure  is  zero.  However,  in  steam  engineering  work  the  word 
vacuum  refers  to  any  space  in  which  the  pressure  is  less  than 
atmospheric  pressure.  For  this  reason,  the  reduced  pressure  in  a 
condenser  is  called  a  vacuum. 

The  vacuum  in  a  condenser  may  be  measured  by  means  of  a 
mercury  column  or  by  means  of  a  gage  constructed  somewhat 
like  a  pressure  gage  but  marked  to  read  pressures  less  than  that 
of  the  atmosphere.  The  mercury  column  is  the  more  accurate 
method  and  is  generally  used  where  the  pressure  in  the  condenser 
is  very  low. 

A  device  for  measuring  vacuum  by  means  of  a  mercury  column 
is  illustrated  in  Fig.  153.  In  this  device  a  glass  tube  about  80 
inches  long  is  bent  into  a  U-shape,  and  is  filled  about  half  full  of 
mercury.  One  branch  of  the  glass  U-tube  is  connected  to  the 


244 


STEAM  ENGINES 


space  in  which  the  vacuum  is  to  be  measured,  the  other  branch 
being  left  open  so  that  it  is  under  atmospheric  pressure.  As  the 
pressure  is  reduced  in  the  space  into  which  the  U-tube  is  connect- 
ed (in  this  case,  a  condenser),  the  mercury  will  rise  in  that  branch 
to  a  height  A,  corresponding  to  the  difference  in  pressure  on  the 
surfaces  of  the  mercury  in  the  two  branches  of  the  U-tube. 

The  amount  of  the  vacuum  is  usually  expressed  in  inches  of 
mercury,  or  simply  "inches,"  and  is  the  difference  in  height  of 
mercury  in  the  two  branches  of  the  U-tube.  Thus,  if  the  height 
A  in  Fig.  153  is  20  inches,  the  vacuum  amounts  to  20  inches  of 
mercury,  or  is  said  to  be  "20  inches."  It  should  be  remembered 
that  the  height  of  the  mercury  column  indicates  the  reduction  of 
pressure,  and  not  the  actual  pressure  existing  in  the  condenser.  A 


FIG.   153. 

vacuum  of  20  inches  means  that  the  pressure  has  been  reduced 
enough  to  support  a  column  of  mercury  20  inches  high.  Since 
a  column  of  mercury  1  inch  high  is  equivalent  to  a  pressure  of 
.49  Ib.  per  sq.  in.,  20  inches  corresponds  to  a  reduction  of  pressure 
of  20  X  .49  =  9.8  Ib.  per  sq.  in.  below  atmospheric  pressure. 
Before  the  pressure  still  existing  in  the  condenser  can  be  found, 
it  is  necessary  to  know  the  pressure  of  the  atmosphere.  If  the 
atmospheric  pressure  is  14.7  Ib.  per  sq.  in.,  a  vacuum  of  20  inches 
leaves  a  pressure  of  14.7  —  9.8  =  4.9  Ib.  per  sq.  in.  If  the 
barometer,  which  measures  the  atmospheric  pressure,  reads 
28  inches,  then  20  inches  of  vacuum  leaves  a  pressure  of  28  —  20 
=  8  inches  of  mercury,  or  8  X  .49  =  3.92  Ib.  per  sq.  in. 

It  is  seen  from  the  above  discussion  that  a  statement  to  the 
effect  that  the  vacuum  carried  by  a  condenser  is  a  certain  number 
of  inches  does  not  always  mean  the  same  thing,  because  of  varia- 


CONDENSING  APPARATUS  245 

tion  in  the  atmospheric  pressure.  Thus,  a  vacuum  of  22.5  inches 
at  a  place  5280  feet  above  sea  level  is  as  near  a  perfect  vacuum  as 
28  inches  at  New  York,  wh^gh  is  at  sea  level.  In  the  first  men- 
tioned place  24.5  inches  would  be  a  perfect  vacuum,  while  at 
sea  level  30  inches  would  be  a  perfect  vacuum.  Vacuum  gages 
of  all  kinds  are  practically  always  marked  to  read  in  inches  of 
mercury  to  correspond  with  the  U-tube  described  above. 

In  the  operation  of  a  condenser  there  will  always  be  some 
pressure  in  the  condenser  due  to  the  presence  of  water  vapor  and 
air,  both  of  which  exert  a  pressure.  The  amount  of  this  pressure 
will  depend  upon  the  temperature  of  the  condensate  and  the 
temperature  and  quality  of  the  air  present  in  the  exhaust  steam. 
Air  enters  the  boiler  in  the  feed  water  and  passes  into  the  piping 
system  with  the  steam.  It  also  leaks  into  the  low-pressure 
cylinder  of  the  engine  around  the  piston  rod.  Leaks  in  the 
condenser  itself  and  exhaust  piping,  both  of  which  are  under  a 
pressure  less  than  that  of  the  atmosphere,  also  account  for  the 
presence  of  some  air  in  the  condenser.  Whatever  air  is  present 
adds  its  pressure  to  that  of  the  water  vapor,  so  that  the  total 
pressure  in  the  condenser  is  equal  to  the  sum  of  the  vapor  pressure 
and  the  air  pressure. 

The  vapor  pressure  depends  upon  the  temperature  of  the 
condensate  and  is  the  same  pressure  as  that  shown  in  the  steam 
table  as  corresponding  to  the  various  temperatures.  For  exam- 
ple, if  the  temperature  of  the  condensate  is  101.83°F.,  the  vapor 
pressure  in  the  condenser  is  1  Ib.  per  sq.  in.  which  corresponds  to 

-  ,0  or  2.04  inches  of  mercury  or  to  a  vacuum  of  30  —  2.04  = 
.4y 

27.96  in.  (atmospheric  pressure  being  14.7  Ib.  per  sq.  in.).  If 
the  temperature  of  the  condensate  was  126.15°F.,  the  vapor 
pressure  would  be  2  Ib.  per  sq.  in.  corresponding  to  a  vacuum  of 

2 

30 T^  or  25.92  in.     It  must  be  remembered  that  the  pressure 

.~ty 

of  the  air  in  the  condenser  acts  in  addition  to  the  vapor  pressure, 
so  that  if,  for  example,  there  were  enough  air  present  in  the  con- 
denser, in  the  last  case  mentioned  above,  to  create  a  pressure  of 
1  Ib.  per  sq.  in.  the  total  pressure  in  the  condenser  would  be  3 

O 

Ib.  instead  of  2  Ib.  and  the  vacuum  would  be  30  — ^  or  23.87 
in. 


246 


STEAM  ENGINES 


It  is  necessary  continually  to  remove  the  air  and  condensate 
as  they  would  soon  accumulate  and  destroy  the  vacuum.  But 
no  matter  how  perfect  the  condensing  and  pumping  apparatus 
there  will  always  be  some  vapor  pressure  due  to  the  temperature 
of  the  condensate,  and  there  will  always  be  more  or  less  air  enter- 
ing the  condenser  with  the  exhaust  steam.  The  more  perfect 


B 


FIG.  154. 

the  apparatus  for  removing  these,  and  the  colder  the  condensing 
water,  the  higher  will  be  the  degree  of  vacuum  that  may  be 
carried. 

Forms  of  Condensing  Apparatus. — Condensers  may  be  divided 
into  two  different  classes  or  types;  namely,  those  in  which  the 
steam  is  condensed  in  direct  contact  with  water,  the  condensate 


CONDENSING  APPARATUS  247 

and  condensing  water  mixing  and  leaving  the  condenser  at  the 
same  temperature;  and  those  in  which  the  steam  is  condensed  in 
contact  with  tubes  through  which  or  around  which  the  condensing 
water  flows.  In  the  latter  dfass  the  condensate  and  condensing 
water  are  kept  separate  and  the  condensing  water  leaves  the 
condenser  at  a  somewhat  lower  temperature  than  the  condensate. 
Practically  all  condensers  fall  into  one  or  the  other  of  these  two 
classes  although  there  are  many  varieties  in  each  class. 

Jet  Condenser. — One  of  the  simplest  forms  of  condensers  is  the 
jet  condenser  illustrated  in  Fig.  154.  In  this  condenser,  the 
condensing  water  enters  the  injection  pipe  at  B.  At  the  end  of 
the  injection  pipe  is  an  adjustable  cone-shaped  spray  head  which 
breaks  the  water  into  a  fine  spray.  The  exhaust  steam  entering 
at  A  meets  the  spray  of  water,  condenses,  and  falls  to  the  bottom 
of  the  condensing  chamber  F.  From  the  bottom  of  the  con- 
densing chamber  the  mixture  of  condensing  water,  condensate,  and 
air  is  taken  into  the  steam-driven  wet  air  pump  G  and  forced  out 
through  J  into  the  hot  well.  If  there  should  be  an  excess  of  con- 
densing water,  due  either  to  the  improper  regulation  of  the  spray 
head  or  to  the  failure  of  the  pump  to  remove  the  water  as  fast 
as  it  collects  in  the  bottom  of  the  condensing  chamber,  the  water 
will  rise  in  the  condensing  chamber  until  it  covers  the  spray  head 
and  the  condensation  of  steam  will  be  practically  stopped.  The' 
vacuum  will  then  be  greatly  reduced,  or  broken,  and  the  pressure 
of  the  exhaust  steam  acting  upon  the  surface  of  the  water  will 
force  it  through  the  valves  of  the  pump  and  into  the  hot  well. 
There  would  then  be  no  danger  of  water  flooding  back  into  the 
engine  cylinder  and  wrecking  it. 

With  this  type  of  condenser  it  is  not  necessary  to  have  the 
supply  of  condensing  water  under  pressure  as  the  vacuum  in  the 
condensing  chamber  will  draw  the  condensing  water  through  the 
injection  pipe  unless  the  supply  is  more  than  about  15  feet  below 
the  condenser.  However,  if  the  condenser  draws  in  its  own  supply 
of  condensing  water  there  must  be  some  means  provided  for 
creating  a  vacuum  to  start  the  condenser.  This  may  be  done 
by  starting  the  pump  or  by  providing  an  auxiliary  injection  of 
water  under  pressure  so  that  the  first  steam  to  come  from  the 
engine  may  be  condensed  and  thus  create  a  vacuum  for  starting 
the  water  through  the  spray  head. 

The  amount  of  vacuum  that  may  be  maintained  in  a  jet 
condenser  depends  upon  the  temperature  of  the  water  in  the 


248 


STEAM  ENGINES 


bottom  of.  the  condenser,  the  amount  of  air  carried  into  the  con- 
denser by  the  condensing  water  and  steam  and  upon  the  tightness 
of  the  valves  and  joints. 

Siphon  Condensers. — A  type  of  jet  condenser  known  as  the 
siphon  condenser  is  illustrated  in  Fig.  155.  In  this  condenser 
the  cooling  water  enters  at  the  side  through  the  pipe  A  and  over- 
flows the  edges  of  the  cone- 
shaped  nozzle  H.  This  causes 
the  water  to  form  a  hollow  cone 
which  becomes  a  solid  stream  in 
passing  through  the  throat"  of  the 
nozzle  at  E.  The  amount  of 
water  passing  through  the  con- 
denser is  regulated  by  means  of 
the  valve  D.  The  exhaust  steam 
enters  the  condenser  at  B  and  is 
given  a  downward  direction  by 
means  of  the  goose  neck  C.  It 
then  comes  in  contact  with  the 
hollow  cone  of  water  and  is  con- 
densed. The  mixture  of  condensed 
steam  and  condensing  water,  to- 
gether with  any  entrained  air, 
passes  through  the  contracted 
neck  of  the  cone  at  E  with  a 
high  velocity  and  is  discharged 
into  the  tail  pipe  at  Fy  the  lower 
end  of  which  is  below  the  surface 
of  the  water  in  the  hot  well. 

When  in  operation,  with  a 
vacuum  in  the  condenser,  the  tail 
pipe  filled  with  water,  and  the  end  of  the  tail  pipe  sealed  by  the 
water  in  the  hot  well,  which  is  under  atmospheric  pressure,  the 
condenser  acts  like  a  barometer.  The  atmospheric  pressure  on 
the  surface  of  the  water  in  the  hot  well  would  therefore  force  the 
water  in  the  tail  pipe  to  a  height  of  34  feet  which  corresponds  to 
atmospheric  pressure.  For  this  reason,  it  is  necessary  that  the 
tail  pipe  be  at  least  34  feet  long  in  order  to  prevent  the  water 
from  backing  through  the  condenser  and  into  the  cylinder  of 
the  engine.  The  force  of  the  water  passing  through  the  con- 
tracted neck  of  the  nozzle  will  balance  several  pounds  of  atmos- 


FIG.   155. 


CONDENSING  APPARATUS 


249 


pheric  pressure  so  that  the  length  of  the  tail  pipe  might  be  made 
a  little  less  than  34  feet  but,  to  guard  against  any  possibility  of 
an  accident,  the  tail  pipe  is  made  at  least  34  feet  long. 

The  condensing  water  may  be  pumped  into  the  condenser  under 
pressure  or  the  vacuum  in  the  condenser  may  draw  it  in  if  it  does 
not  have  to  lift  it  more  than  about  15  feet.  It  is  possible  to  use 
very  muddy  or  dirty  water  in  this  type  of  condenser  as  it  is  not 


FIG.  156. 


very  likely  to  become  stopped  up.  However,  should  a  stoppage 
occur  or  should  the  condenser  fail  to  operate  for  any  reason, 
pressure  will  accumulate  in  the  condenser  until  it  opens  the  auto- 
matic relief  valve  shown  at  G  in  Fig.  155,  and  the  engine  will 
then  exhaust  into  the  atmosphere. 

Barometric  Condenser.- — Although  the  siphon  condenser  just 
described  might  be  classed  as  a  barometric  condenser  inasmuch 
as  there  is  barometric  action  in  the  tail  pipe,  the  name  barometric 
condenser  is  usually  reserved  for  that  class  of  condensers  in  which 
there  is  no  injector  or  ejector  action  through  a  contracted  tube. 

25 


250  STEAM  ENGINES 

Such  a  condenser  as  this  is  illustrated  in  Fig.  156,  the  tail  pipe 
being  left  off  to  permit  a  larger  illustration  being  used. 

In  this  type  of  condenser  the  exhaust  steam  is  brought  into 
contact  with  a  spray  of  cold  water  and  is  condensed.  The  mixture 
of  condensed  steam  and  water  then  flows  by  gravity  through  a 
tail  pipe,  the  lower  end  of  which  is  sealed  by  the  water  in  the  hot 
well.  In  operation,  the  tail  pipe  of  this  condenser  acts  as  a  true 
barometric  tube  with  the  water  standing  in  it  at  a  height  corre- 
sponding to  the  vacuum  carried  in  the  condenser.  As  more  water 
is  added  to  the  column  at  the  top,  a  like  amount  flows  out  at  the 
bottom. 

The  condensing  water  enters  the  injection  pipe  at  B  and  is 
broken  into  a  spray  by  means  of  the  cone  F.  This  cone  is  sus- 
pended from  a  coil  spring  above  so  that  the  nozzle  opening  is 
automatically  adjusted  by  the  water  pressure.  The  exhaust 
steam  enters  at  A  and  divides  into  two  parts,  one  part  passing 
directly  into  the  condensing  chamber  D  where  it  meets  the  spray 
of  cold  water  and  the  other  part  passing  downward  through  the 
annular  space  E  and  then  upward  into  the  condensing  chamber. 
Any  air  that  is  not  entrained  in  the  water  passing  into  the  tail 
pipe  and  also  any  uncondensed  vapor  pass  up  through  the  center 
of  the  spray  nozzle,  which  is  hollow,  and  into  the  air  cooling 
chamber  at  the  top.  The  air  is  cooled  and  the  vapor  condensed 
by  an  injection  of  cold  water  through  the  pipe  K.  The  air  is 
then  drawn  off  through  the  air  pipe  H  by  means  of  a  dry  air 
pump.  Since  the  air  and  water  move  in  opposite  directions,  this 
type  of  condenser  is  known  as  a  countercurrent  condenser. 

Surface  Condensers. — This  class  of  condensers  includes  all 
those  in  which  the  steam  and  condensing  water  are  kept  separate 
and  in  which  the  steam  is  condensed  on  a  metal  surface.  These 
condensers  usually  consist  of  a  shell  which  contains  a  large  number 
of  small  tubes,  with  the  condensing  water  on  the  inside  of  the 
tubes  and  the  steam  on  the  outside,  although  in  a  number  of 
makes  of  surface  condensers  the  condensing  water  is  on  the 
outside  and  the  steam  on  the  inside  of  the  tubes.  The  tubes  are 
usually  made  of  brass  because  this  metal  conducts  heat  better 
than  iron. 

A  typical  example  of  a  surface  condenser  is  illustrated  in  Fig. 
157  which  shows  a  Wheeler  condenser  with  wet  air  pump  and 
circulating  pump  for  forcing  the  condensing  water  through  the 
condenser.  This  condenser,  which  has  a  rectangular  cross  section, 


CONDENSING  APPARATUS 


251 


252  STEAM  ENGINES 

consists  of  a  cast-iron  shell  or  case  containing  a  large  number  of 
closely  spaced  small  seamless  drawn  brass  tubes  through  which 
the  condensing  water  circulates.  The  tubes  have  considerable 
length  compared  with  their  diameter  and  they  are  therefore 
supported  at  their  middle  points  to  prevent  their  sagging.  They 
are  fastened  into  the  tube  sheets  at  their  ends  by  means  of  a 
stuffing  box  made  of  a  brass  ferrule  with  packing.  This  con- 
struction is  used  so  that  the  tubes  will  be  free  to  expand  and 
contract  without  leaking  and  so  they  may  be  easily  removed. 

The  space  in  the  shell  between  the  tube  sheets  and  the  heads 
of  the  condenser  forms  the  condensing  water  compartments. 
The  water  compartment  at  the  right  of  Fig.  157  is  divided  by  a 
baffle  plate  which  causes  the  water  to  flow  in  one  direction  through 
the  bottom  set  of  tubes  and  to  flow  in  the  opposite  direction 
through  the  top  set,  the  water  being  forced  through  the  tubes 
by  the  pump  shown  at  the  right  of  Fig.  157. 

The  exhaust  steam  enters  the  condenser  through  the  large 
opening  shown  at  the  top  of  the  shell.  Upon  entering,  it  strikes 
the  baffle  plate  shown  just  over  the  tubes,  which  prevents  it  from 
striking  directly  on  the  tubes  and  distributes  it  over  a  greater 
area  of  cooling  surface.  In  this  way  the  entire  tube  surface  is 
made  more  effective.  The  condensation  of  steam  begins  on  the 
surface  of  the  upper  row  of  tubes  and  continues  as  the  steam 
passes  down  among  the  tubes,  the  condensate  dropping  to  the 
bottom  of  the  condenser  shell.  The  condensing  water  entering 
at  the  bottom,  increases  in  temperature  as  it  flows  towards  the 
top,  until  its  temperature  upon  leaving  is  practically  the  same  as 
that  of  the  exhaust  steam.  The  condensate,  together  with  any 
entrained  air,  is  drawn  into  the  wet  vacuum  pump  shown  at  the 
left  of  Fig.  157  and  is  pumped  out  of  the  condenser. 

In  operating  a  condenser  of  the  kind  described  above  particular 
care  should  be  used  to  see  that  exhaust  steam  is  not  turned  into 
the  condenser  unless  there  is  water  in  the  tubes  as  the  tube  pack- 
ings are  liable  to  be  destroyed  and  the  shell  or  tube  sheets  cracked, 
particularly  if  cold  water  is  admitted.  Sudden  or  large  changes 
of  temperature  should  be  avoided  as  they  are  apt  to  injure  the 
tubes  or  to  cause  them  to  leak.  The  water  chambers  should  be 
examined  frequently  to  see  that  no  foreign  matter  is  collecting 
and  stopping  the  tubes  as  this  will  lower  the  vacuum  and  increase 
the  duty  of  the  tubes  and  circulating  pump.  The  outside  of  the 
tubes  can  be  kept  clean  and  efficient  by  introducing  about  a 


CONDENSING  APPARATUS  253 

gallon  of  kerosene  with  the  exhaust  steam  once  a  week  and  just 
before  shutting  down.  This  will  free  the  tubes  from  oil  carried 
over  in  the  exhaust  steam. 

High  Vacuum  Condensers.— Any  of  the  condensing  systems 
described  above  are  well  adapted  for  use  with  steam  engines  be- 
cause a  steam  engine  operates  at  its  best  commercial  economy 
with  a  vacuum  of  about  26  in.  (referred  to  a  30-in.  barometer) 
and  any  of  these  condensers  will  produce  a  vacuum  of  from  24  to 
26  in.  If  a  steam  engine  is  operated  at  a  higher  vacuum  than 
about  26  in.,  cylinder  condensation  becomes  excessive  because  of 
the  large  size  of  the  cylinder  made  necessary  by  the  largely  in- 
creased volume  of  the  steam  at  very  low  pressures.  The  in- 
creased size  of  the  piston  also  increases  the  friction  so  that  the 
loss  from  these  two  sources  more  than  balances  the  gain  from 
increased  expansion  of  the  steam. 

A  steam  turbine,  on  the  other  hand,  will  show  a  considerable 
increase  in  efficiency  from  the  use  of  vacua  above  26  in.  This  is 
largely  because  the  steam  in  passing  through  the  turbine  experi- 
ences a  gradual  drop  in  temperature  and  the  parts  of  the  turbine 
are  not  subjected  to  a  great  range  of  temperature  as  in  a  steam 
engine,  hence  there  is  not  much  condensation  of  steam  in  the 
turbine.  For  the  reasons  just  mentioned  condenser  manufac- 
turers have  made  considerable  improvement  in  condensing  sys- 
tems since  the  introduction  of  the  steam  turbine.  The  principal 
improvements  in  securing  higher  vacua  were  the  use  of  a  separate 
dry  vacuum  pump  for  removing  the  air  and  noncondensable 
vapors,  and  also  the  providing  of  means  for  cooling  the  air  before 
it  enters  the  air  pump  so  that  the  volume  to  be  handled  will  not 
be  so  large. 

Choice  of  a  Condenser. — The  choice  of  a  condenser  for  a  steam 
engine  depends  largely  upon  the  kind  and  cost  of  the  available 
supply  of  condensing  water.  Where  there  is  a  plentiful  and 
cheap  supply  of  good  condensing  water  which  is  also  suitable  for 
feeding  into  the  boiler,  some  good  type  of  jet  condenser  will  gen- 
erally be  found  most  desirable.  If  there  is  sufficient  overhead 
room,  a  siphon  or  barometric  condenser  will  be  found  most  desir- 
able and  least  expensive.  In  this  connection,  it  must  be  remem- 
bered that  these  condensers  may  often  be  located  to  advantage 
outside  the  building.  A  condenser  of  the  siphon  or  ejector  type 
will  also  use  to  advantage  a  very  dirty  or  muddy  condensing  water 
which  would  not  be  at  all  suitable  for  feeding  into  the  boiler.  If 

26 


254  STEAM  ENGINES 

this  is  done,  however,  the  feed  water  must  be  heated  in  some  other 
way,  but  this  can  usually  be  done  by  means  of  the  exhaust  from 
the  steam-driven  pumps  and  other  auxiliaries  around  the  power 
plants. 

When  the  supply  of  condensing  water  is  not  suitable  for  feeding 
into  the  boiler,  a  surface  condenser  should  be  used  because  then 
the  same  feed  water  may  be  used  over  and  over  and  only  enough 
additional  feed  water  need  be  supplied  to  make  up  the  losses  by 
leakage  and  other  sources  of  waste.  However,  the  condensing 
water  used  in  a  surface  condenser  should  not  be  so  dirty  as  to 
cause  stoppage  of  the  tubes  nor  should  it  contain  enough  mineral 
matter  to  give  trouble  from  incrustation  as  the  tubes  of  a  surface 
condenser  are  so  small  and  so  numerous  that  cleaning  them  is 
difficult  and  expensive.  On  the  other  hand,  not  much  trouble 
from  incrustation  is  to  be  expected  because  the  condensing  water 
does  not  reach  a  high  enough  temperature  to  cause  mineral  sub- 
stances to  deposit  very  fast  and  also  because  the  velocity  of  the 
water  through  the  tubes  is  high  enough  to  retard  the  deposit  of 
mineral  matter. 


CHAPTER  XIX 
LUBRICATION 

Friction. — Every  bearing  in  every  piece  of  machinery  in  a 
power  plant  produces  friction.  Friction  in  bearings  generates 
heat,  which  is  one  form  of  energy,  and,  as  there  is  no  way  of  utiliz- 
ing this  heat,  it  is  lost.  For  this  reason,  friction  in  bearings 
represents  a  direct  loss.  The  continual  overcoming  of  friction 
requires  power,  and  it  is  for  this  reason  that  some  power  is 
required  to  run  a  steam  engine  even  when  it  is  carrying  no  load 
except  the  load  caused  by  the  friction. 

The  power  lost  in  overcoming  friction  in  a  power  plant  depends 
upon  the  number,  kind,  and  condition  of  the  bearings.  It  is 
seldom  less  than  5  per  cent,  of  the  total  power  generated  and  it  is 
sometimes  as  much  as  30  per  cent.  The  power  wasted  by  friction 
in  a  steam  engine  will  amount  to  from  5  to  20  per  cent,  of  the  total 
power  of  the  engine,  with  about  10  per  cent,  representing  the 
average  for  an  engine  with  good  bearings  well  lubricated. 

The  losses  due  to  friction  are  not  only  the  loss  of  power  but  they 
include  also  the  repairs  and  depreciation  due  to  wear  on  the  bear- 
ings, guides,  piston  rod  and  packing,  piston,  and  other  rubbing 
surfaces. 

The  losses  mentioned  above  may  be  reduced  considerably  by 
using  a  sufficient  quantity  of  the  proper  kind  of  lubricant.  The 
selection  of  the  kind  of  lubricant  is,  therefore,  a  very  important 
problem  and  a  change  in  the  kind  of  lubricant  used  may  often 
result  in  the  large  increase  in  the  economy  of  operation  of  the  en- 
gine. The  choosing  of  the  proper  lubricant  to  use  in  any  particu- 
lar bearing  is  not  a  very  simple  matter,  as  there  is  such  a  large 
variety  of  lubricants  on  the  market.  The  selection  of  a  lubricant 
for  any  particular  purpose  should,  therefore,  be  undertaken  only 
after  the  fundamental  principles  of  lubrication  and  the  necessary 
qualities  of  the  lubricant  are  thoroughly  understood. 

Lubrication. — If  you  should  look  through  a  microscope  at  the 
surface  of  a  polished  shaft  it  would  appear  to  be  rough,  even 
though  as  far  as  the  naked  eye  can  see,  the  shaft  is  perfectly 
27  255 


256 


STEAM  ENGINES 


smooth.  After  looking  at  such  a  surface  through  a  microscope 
and  seeing  its  roughness,  it  will  be  realized  that  when  two  such 
surfaces  are  brought  together  the  elevations  and  depressions  of 
one  surface  will  interlock  with  those  of  the  other  surface,  as 
shown  magnified  in  Fig.  158,  and  that  considerable  force  will  be 
required  to  move  one  surface  over  the  other.  In  other  words, 


FIG.  158. 

there  will  be  considerable  friction  between  the  two  surfaces.  The 
friction  in  this  case  would  arise  from  two  sources :  first,  from  mov- 
ing the  irregularities  on  the  surface  of  the  journal  into  and  out  of 
the  irregularities  on  the  bearing  surface,  that  is,  the  friction  due 
to  the  unevenness  of  the  metal  surfaces;  second,  the  friction  due 
to  the  cutting  action  of  the  metal  surfaces  upon  one  another. 


FIG.   159. 

Even  though  the  irregularities  in  the  metal  surfaces  are  invis- 
ible to  the  naked  eye,  it  will  be  realized  that  the  amount  of  fric- 
tion between  the  surfaces  is  enormous,  especially  if  the  bearing 
supports  considerable  weight  or  runs  at  high  speed. 

Principles  of  Lubrication. — The  object  of  lubrication  is  to  place 
a  thin  film  of  oil  between  the  metal  surfaces  so  they  will  not  come 


LUBRICATION 


257 


in  contact  with  each  other  while  running.  If  this  object  is  accom- 
plished the  metal  surfaces  will  not  touch  but  the  journal  will 
"float  on  oil"  and  the  friction  will  be  enormously  reduced.  This 
condition  is  illustrated  in  Fig.  159  which  shows  the  two  surfaces 
of  Fig.  158  but  with  a  film  of  oil  between  them. 

In  order  to  secure  a  film  of  oil  between  bearing  surfaces  some 
provision  must  be  made  so  that  the  surfaces  will  not  scrape  off  the 
film  of  oil  but  rather  that  the  moving  surface  will  be  made  to  ride 
upon  the  particles  of  oil.  When  the  surfaces  to  be  lubricated  are 


FIG.  160. 

flat,  as  is  the  case  with  the  crosshead  or  piston  of  an  engine, 
the  ends  of  the  crosshead  or  piston  should  be  slightly  beveled  so 
that  there  are  no  sharp  edges.  If  the  edges  are  left  sharp  the  film 
of  oil  will  be  scraped  off  and  there  will  be  metallic  contact  between 
the  bearing  surfaces.  This  is  illustrated  in  Fig.  160  which  shows 
the  end  of  a  piston  slightly  beveled  so  that  oil  may  collect  under 
it  and  keep  the  surfaces  apart.  For  the  same  reason,  the  edges  of 
piston  rings  should  be  slightly  beveled  instead  of  left  with  sharp 
edges. 

With  a  round  shaft  which  turns  in  a  bearing,  such  as  the  main 


258 


STEAM  ENGINES 


bearing  of  an  engine,  the  same  result  is  automatically  secured 
through  the  fact  that  the  bearing  which  surrounds  the  shaft 
always  has  a  slightly  larger  diameter  than  the  shaft.  This  leaves 
a  small  clearance  at  the  top  and  sides  of  the  bearing,  as  illus- 
trated in  Fig.  161,  which  is  somewhat  exaggerated  in  order  to 
show  the  clearance  more  plainly.  The  clearance  at  the  sides  of 
the  bearing  permits  the  oil  to  be  drawn  in  between  the  journal 
and  bearing  and  thus  form  a  film  between  them.  The  oil  adher- 
ing to  the  surface  of  the  journal  not  only  causes  it  to  be  drawn 
into  the  bearing  but  also  causes  it  to  be  drawn  out  at  the  other 
side.  With  large  bearings  which  are  supplied  with  oil  at  the 


FIG.  161. 

center  of  the  top  it  is  necessary  to  provide  diagonal  oil  grooves  in 
the  bearing  surface  so  that  the  oil  may  be  spread  to  all  parts  of  it. 
There  will  always  be  some  friction,  even  with  a  well-lubricated 
bearing  in  which  a  film  of  oil  is  maintained,  but  in  this  case  the 
friction  is  largely  a  fluid  friction  instead  of  the  friction  of  metal- 
lic surfaces  in  contact.  When  the  oil  has  been  carried  into  the 
bearing  it  comes  in  contact  with  the  bearing  surface  and  adheres 
or  sticks  to  it.  Since  the  bearing  is  stationary,  the  particles  of 
oil  next  to  its  surface  will  also  be  stationary  and  the  particles  of 
oil  which  are  next  to  the  revolving  journal  will  be  in  motion  due 
to  these  particles  clinging  to  the  journal.  The  velocity  of  the 
particles  of  oil  in  the  film,  therefore,  varies  all  the  way  from  no 


LUBRICATION 

velocity  on  one  side  of  the  film  to  the  velocity  of  the  journal  on  the 
other  side  of  the  film.  l\  is  this  relative  motion  of  the  particles 
of  oil  among  themselves  that  is  largely  the  cause  of  friction  in  a 
well-lubricated  bearing,  and  this  friction  is  more  of  a  fluid  friction 
than  a  friction  between  metal  surfaces. 

Characteristics  of  Oil. — A  journal  picks  up  a  film  of  oil  and 
carries  it  into  the  bearing  by  reason  of  the  adhesiveness  of  the 
oil,  that  is,  its  ability  to  adhere  to  the  surface  of  the  journal.  The 
quantity  of  oil  that  will  be  drawn  into  the  bearing  in  this  way 
depends  upon  the  viscosity  of  the  oil.  A  homely  illustration 
of  this  action  is  the  following:  If  you  stick  your  finger  into  a  cup 
of  thick  molasses  and  then  withdraw  it,  a  large  quantity  of  the 
molasses  will  adhere  to  your  finger  and  be  withdrawn  with  it.  If 
you  try  the  same  with  a  cup  of  water,  only  a  very  small  quantity  of 
water  will  adhere  to  your  finger  and  be  withdrawn  with  it,  be- 
cause the  water  is  so  much  thinner  and  more  fluid  than  the  molas- 
ses. When  an  oil  is  thick  or  does  not  flow  readily  it  is  said  to  be 
viscous,  or  its  viscosity  is  high.  If  an  oil  is  thin  or  flows  readily 
its  viscosity  is  low.  In  other  words,  the  viscosity  of  oil  refers 
to  its  "body." 

Since  the  work  which  a  lubricating  oil  is  called  upon  to  do  is 
keeping  the  metal  surfaces  of  the  bearing  apart  by  means  of  a 
film  of  oil,  the  conditions  under  which  the  oil  is  to  be  used  deter- 
mine the  proper  viscosity  or  body  which  the  oil  should  have. 
These  conditions  are  the  speed  of  the  moving  parts,  the  weight  on 
the  bearing  and  its  running  temperature. 

If  an  oil  is  too  low  in  viscosity  it  will  permit  metallic  contact 
between  the  surfaces  to  be  lubricated,  with  a  consequent  loss  of 
power,  and  wear  on  the  bearing.  On  the  other  hand,  if  an  oil  too 
high  in  viscosity  is  used,  there  will  be  unnecessary  fluid  friction, 
with  a  consequent  loss  of  power.  The  more  viscous  the  oil,  the 
greater  the  pressure  which  can  be  sustained  without  metallic  con- 
tact. The  viscosity  of  the  oil  should,  therefore,  be  in  proportion 
to  the  pressure  on  the  bearing.  Heavy,  slow-moving  bearings 
require  an  oil  of  high  viscosity;  light,  swift-moving  bearings  re- 
quire a  thin  oil,  or  an  oil  of  low  viscosity. 

The  viscosity  of  an  oil  is  different  at  different  temperatures; 
therefore,  in  selecting  an  oil  to  be  used,  the  temperature  at  which 
the  bearing  runs  must  be  considered.  Most  of  the  friction  in  a 
well-lubricated  bearing  is  internal  or  fluid  friction,  as  explained 
before,  and  this  friction  causes  the  temperature  of  the  bearing  to 


260  STEAM  ENGINES 

be  somewhat  higher  than  the  room  temperature.  Since  a  heavy 
oil  has  more  internal  friction  than  a  light  one,  the  viscosity  should 
be  only  high  enough  to  maintain  the  film  of  oil  at  the  temperature 
at  which  the  bearing  runs. 

Testing  Oils. — From  the  above  discussion  of  lubrication  it  is 
apparent  that  one  of  the  most  important  qualities  of  a  lubricating 
oil  is  its  viscosity.  The  measurement  of  viscosity  of  lubricating 
oils  is  in  a  certain  sense  unsatisfactory  because  the  results  ob- 
tained with  the  different  instruments  which  are  available  for  this 
purpose  do  not  agree  among  themselves.  For  this  reason  the 
make  of  instrument  used  in  making  the  test  should  be  stated  in 
quoting  the  viscosity. 

One  of  the  most  common  instruments  used  in  testing  viscosity 
is  the  Saybolt  viscosimeter.  It  consists  of  a  tall  pipette  of 
small  diameter  surrounded  with  a  jacket  which  may  be  used  for 
maintaining  the  oil  at  any  desired  temperature  during  the  test. 
The  test  is  made  by  filling  the  pipette  to  a  certain  point  with  the 
oil  whose  viscosity  is  to  be  measured  and  noting  the  time  in  sec- 
onds which  the  oil  takes  to  run  out  of  the  pipette.  If  it  is  desired 
to  find  the  specific  viscosity,  that  is,  the  viscosity  of  the  oil  com- 
pared with  that  of  water,  this  may  be  done  by  dividing  the  time 
which  the  oil  takes  to  run  out  by  the  time  it  takes  an  equal 
volume  of  water  to  run  out.  It  should  be  particularly  noted 
that  the  viscosity  of  oil  varies  greatly  with  its  temperature. 
The  temperature  at  which  the  test  is  made  should,  therefore,  be 
stated  and,  to  be  of  practical  use,  the  temperature  during  the  test 
should  be  as  near  as  possible  to  the  temperature  at  which  the  oil 
is  to  be  used. 

Gumming  Test. — A  gumming  test  is  of  importance  because  it 
indicates  the  extent  to  which  the  oil  has  been  refined  and  also  the 
extent  to  which  the  oil  may  be  expected  to  change,  due  to  oxida- 
tion, when  in  use. 

The  gumming  test  may  be  made  by  putting  a  small  quantity  of 
the  oil  to  be  tested  in  a  small  glass  vessel,  such  as  a  cordial  glass, 
and  then  mixing  with  it  an  equal  quantity  of  nitrosulphuric  acid. 
A  properly  refined  oil  will  show  little  if  any  change,  but  a  poorly 
refined  oil  will  show  the  separation  of  large  quantities  of  material 
of  dark  color,  due  to  the  oxidation  of  tarry  matter  contained  in  the 
lubricant.  Oils  which  contain  a  large  percentage  of  tar  absorb 
the  most  oxygen  and  are,  therefore,  mildly  drying. 

Flash  and  Fire  Tests. — These  tests  have  nothing  to  do  with  the 


LUBRICATION  261 

lubricating  qualities  of  an  oil  but  they  give  an  indication  of  its 
safety  and  for  this  reason  they  are  important. 

The  flash  test  is  made  by  heating  the  oil  slowly  in  a  vessel  sur- 
rounded by  a  proper  bath  and  noting  the  temperature  at  which 
a  flame  passed  over  the  surface  of  the  oil  will  ignite  the  vapors 
arising  from  the  oil.  The  fire  test  is  made  by  continuing  to  heat 
the  oil  slowly  and  noting  the  temperature  at  which  the  vapors, 
when  ignited,  will  continue  to  burn. 

Acid  Test. — During  the  process  of  refining,  oil  is  agitated  with 
sulphuric  acid  for  the  purpose  of  removing  the  tarry  matter,  and 
this  acid  must  be  practically  all  removed  before  the  oil  is  put  on 
the  market  as  any  free  acid  in  it  is  apt  to  corrode  the  bearings  in 
which  it  is  used. 

Acid  may  be  readily  detected  by  thoroughly  mixing  a  small 
quantity  of  the  oil  with  an  equal  amount  of  pure  warm  water. 
The  mixture  should  then  be  tested  with  neutral  litmus  paper. 
If  acid  is  present,  the  paper  will  immediately  turn  red.  The 
sensitiveness  of  the  paper  may  be  greatly  increased  by  introduc- 
ing it  into  the  fumes  of  nitric  or  hydrochloric  acid  until  it  becomes 
partly  red.  After  it  is  dried  and  part  of  its  surface  is  immersed 
in  the  oil  to  be  tested  the  very  smallest  amount  of  acid  will  turn 
the  part  of  the  paper  immersed  a  shade  darker  than  the  rest. 

It  should  be  appreciated  by  the  practical  man  that  the  tests 
of  lubricating  oils  give  only  an  approximate  idea  of  the  qualities 
of  the  oil.  In  fact,  no  rigid  directions  can  be  given  for  the  choice 
of  an  oil  for  a  specific  purpose.  It  is  best  to  try  various  lubri- 
cants which  can  be  purchased  for  any  given  lubricating  problem 
until  one  is  found  which  gives  satisfactory  results  under  actual 
working  conditions.  This  should  then  be  completely  tested  and 
the  results  used  in  buying  oil  for  the  same  purpose  in  the  future. 

Steam -Engine  Lubrication. — The  lubrication  of  a  steam  engine 
presents  two  distinct  and  separate  problems.  First,  there  is  the 
problem  of  lubricating  such  bearings  as  the  main  bearings,  crank 
and  -wrist  pins,  and  other  external  bearings.  The  principles 
governing  the  lubrication  of  these  bearings  have  been  discussed 
in  the  first  part  of  this  chapter.  Second,  there  is  the  problem  of 
lubricating  the  internal  bearings,  such  as  piston,  valves,  piston 
rod,  and  valve  rod. 

The  problem  of  lubricating  the  internal  bearings,  especially  the 
piston  and  valves,  is  far  more  difficult  than  that  of  lubricating  the 
external  bearings.  The  external  bearings  are  more  easily  in- 


262  STEAM  ENGINES 

spected  to  determine  if  they  are  being  properly  lubricated.  If 
they  receive  too  little  oil  the  temperature  of  the  bearing  becomes 
too  high;  if  they  receive  too  much  oil  this  can  be  told  by  the  over- 
flow and  waste  of  oil  from  the  bearing.  The  bearing  surfaces  of 
the  piston  and  valves  are  not  so  easily  inspected,  nor  is  the  fact 
easily  detected  that  too  little  or  too  much  oil  is  being  supplied  or 
that  the  oil  is  being  properly  distributed.  In  general,  a  light  oil 
or  oil  of  low  viscosity  is  used  in  the  external  bearings,  while  the  in- 
ternal bearings  require  a  heavy  oil  or  oil  of  high  viscosity.  Cyl- 
inder oil  should  also  have  a  high  flash  and  burning  point  in  order 
to  prevent  it  from  being  carbonized  by  the  high  temperature  in 
the  cylinder.  This  is  especially  true  when  the  engine  is  supplied 
with  superheated  steam. 

The  best  method  of  carrying  oil  into  a  steam-engine  cylinder 
for  internal  lubrication  is  to  introduce  it  into  the  steam  line  sup- 
plying the  engine.  This  is  accomplished  by  connecting  the  feed 
pipe  from  the  lubricator  to  the  steam  line,  allowing  the  feed  pipe 
to  extend  into  the  center  of  the  steam  line.  By  doing  this,  the 
cylinder  oil,  coming  in  contact  with  the  column  of  steam  flowing 
through  the  steam  line  at  its  point  of  greatest  velocity,  is  broken 
up  into  very  small  particles  and  carried  along  with  the  steam  into 
the  cylinder.  Since  the  steam  comes  in  contact  with  all  internal 
surfaces  requiring  lubrication,  this  method  insures  a  supply  of 
oil  to  all  rubbing  surfaces.  The  point  at  which  the  feed  pipe 
enters  the  steam  line  should  be  on  the  boiler  side  of  the  main  stop 
or  throttle  valve  so  the  spindle  of  the  valve  will  be  lubricated, 
thus  making  easy  the  operation  of  the  valve.  However,  the  feed 
pipe  should  not  enter  at  such  a  point  that  the  spraying  or  atomi- 
zation  of  the  oil  will  be  affected  by  a  steam  separator  or  angles  in 
the  pipe  between  the  point  of  introduction  and  the  steam  chest, 
and  the  oil  should  be  introduced  at  a  point  not  further  than  about 
twelve  feet  back  of  the  throttle  valve. 

Superheated  steam,  which  is  very  dry  and  hot,  is  not  a  good 
carrying  medium  for  cylinder  oil,  and  for  this  reason  the  oil  should 
be  introduced  just  back  of  the  throttle  valve  when  superheated 
steam  is  used. 

Steam  is  sometimes  used  for  jacketing  the  cylinder  and  heads 
of  the  engine  before  entering  the  cylinder.  Under  these  condi- 
tions some  of  the  oil  will  be  deposited  on  the  walls  of  the  passages 
if  it  has  been  introduced  into  the  steam  line  in  front  of  the  throttle 
valve.  To  overcome  this  difficulty  oil  is  introduced  directly  into 


LUBRICATION 


263 


the  valve  chambers  when  the  current  of  steam  passing  through 
the  valves  will  break  up  the  oil  into  fine  particles  and  distribute 
it. 

Lubricators.— Cylinder  oil  is  forced  into  the  steam  line  by 
means  of  a  lubricator.  These  are  of  two  kinds:  hydrostatic 
lubricators  and  mechanically  operated  lubricators. 

A  hydrostatic  lubricator  is  illustrated  in  Fig.  162,  being  shown 
partly  cut  away  so  that  its  internal  construction  and  operation 
may  be  understood.  The  body  of  the  lubricator  contains  cylin- 


FIG.  162. 

der  oil  and  water,  the  oil  remaining  on  top  of  the  water  because  it 
is  lighter.  Water  is  introduced  into  the  lubricator  from  the 
condensation  of  steam  in  the  pipe  3  which  is  connected  to  the 
steam  line  and  the  bulb  4.  It  then  passes  into  the  bottom  of  the 
lubricator  through  the  tube  6,  where  it  exerts  an  upward  pressure 
on  the  oil.  This  pressure  forces  the  oil  out  of  the  lubricator 
through  the  tube  8  which  ends  near  the  top  of  the  lubricator 
where  it  will  be  supplied  with  oil  as  long  as  there  is  any  in  the 
lubricator.  The  oil  passes  through  the  tube  8  to  the  regulating 
valve  9  where  its  rate  of  flow  to  the  cylinder  is  controlled.  It 
forms  into  drops  as  it  passes  through  the  regulating  valve  and 


264  STEAM  ENGINES 

these  drops  rise  through  the  water  in  the  sight  feed  glass  10  and 
through  the  pipe  1 1  into  the  steam  line.  As  the  oil  is  fed  into  the 
steam  line,  its  place  is  taken  by  more  water  which  enters  through 
tube  6;  thus  as  the  amount  of  oil  in  the  lubricator  decreases,  the 
level  of  the  water  rises.  The  amount  of  oil  remaining  in  the 
lubricator  is  indicated  by  the  gage  glass  13.  Since  the  lubricator 
is  connected  to  the  steam  line  at  two  points,  the  steam  pressure  at 
these  two  points  balance  each  other,  and  the  pressure  which 
forces  the  oil  out  comes  from  the  weight  of  the  column  of  water 
in  the  tube  6  and  the  pipe  3.  The  length  of  the  pipe  3  should  be 
at  least  18  inches  so  as  to  provide  sufficient  condensing  surface 
and  a  column  of  water  high  enough  to  force  the  oil  out  of  the 
lubricator. 

When  the  level  of  the  water  in  the  lubricator  reaches  a  point 
near  the  top  of  the  gage  glass  13,  the  lubricator  should  be  refilled 
with  oil.  This  is  done  by  first  closing  the  valve  5  and  the  valve  on 
the  feed  pipe  11  in  order  to  cut  the  steam  pressure  off  of  the 
lubricator  at  both  top  and  bottom.  The  filling  plug  15  of  the 
lubricator  may  then  be  opened,  but  it  must  never  be  opened  while 
there  is  steam  pressure  on  the  lubricator,  as  the  hot  oil  will  then 
be  blown  out  and  may  cause  serious  injury  to  persons  standing 
near;  for  this  reason  every  precaution  must  be  taken  to  close 
valve  5  and  the  valve  on  the  feed  pipe  11,  before  the  filling  plug 
15  is  removed.  The  drain  cock  14  may  now  be  opened  and 
enough  water  drained  out  of  the  lubrictaor  until  its  level  is  near 
the  bottom  of  the  gage  glass  13.  The  oil  may  then  be  poured  in 
through  the  filling  hole  at  the  top  until  the  lubricator  is  full, 
after  which  the  filling  plug  15  is  replaced. 

In  order  to  start  the  lubricator  operating  again  it  is  only 
necessary  to  open  the  valve  5  and  the  valve  on  the  feed  line  11, 
and  then  regulate  the  flow  of  oil  by  means  of  the  regulating 
valve  9. 

The  hydrostatic  lubricator,  described  above,  is  not  automatic 
in  its  action,  since  it  must  be  started  and  stopped  by  hand.  It 
is  difficult  to  maintain  a  constant  feed,  especially  at  a  slow  rate, 
because  the  feed  is  affected  by  the  changes  in  viscosity  of  the  oil 
due  to  changes  in  temperature.  This  is  particularly  true  just 
after  the  lubricator  has  been  refilled. 

These  disadvantages  are  overcome  in  the  mechanically  operated 
lubricator,  one  form  of  which  is  illustrated  in  Fig.  163.  This  type 
of  lubricator  is  constructed  with  single  and  multiple  feed  delivery 


LUBRICATION 


265 


pipes,  the  number  of  feeds  being  limited  to  the  number  of  points 
of  application  demanded  in  each  case. 

In  Fig.  163  the  oil  reservoir  is  shown  at  1,  the  level  of  the  oil 
being  indicated  by  a  glass  gage  which  is  not  shown  in  the  illus- 
tration. The  oil  reservoir  is  provided  with  a  strainer  so  that  fresh 
oil  poured  into  it  will  be  strained  and  any  small  particles  of  solid 
matter  which  might  clog  the  small  oil  passages  strained  out. 
The  oil  plunger  2,  draws  oil  into  the  pump  on  the  suction  stroke 
and  discharges  the  oil  through  the  nozzle  3  on  the  delivery 


FIG.  163. 

stroke.  The  oil  drops  form  around  a  guide  wire  4  and  rise  through 
water  in  the  sight  feed.  They  then  pass  a  nonreturn  valve  5,  and 
are  forced  through  the  check  valve  7,  at  the  extreme  end  of  the  oil 
pipe  6,  into  the  atomizer  8,  located  in  the  steam  pipe  10. 

By  means  of  the  adjusting  nuts  11  and  12,  which  change  the 
stroke  of  the  pump  plunger,  the  oil  supply  can  be  varied  from  the 
smallest  amount,  say  one  drop  in  ten  minutes,  to  practically  the 
full  capacity  of  the  pump  stroke. 

An  atomizer,  such  as  shown  at  8  in  Fig.  163,  is  the  most  efficient 
way  of  spraying  oil  into  and  thoroughly  mixing  it  with  the  steam. 
The  atomizer  is  spoon-shaped  on  its  upper  side  and  has  slots 


266  STEAM  ENGINES 

extending  through  it.  The  steam  strikes  against  the  spoon- 
shaped  upper  surfaces  and  forces  the  oil  through  the  slots  with 
great  velocity,  breaking  it  up  into  very  fine  particles  which  are 
thoroughly  mixed  with  the  steam  flowing  through  the  pipe. 

The  atomized  oil  is  distributed  in  the  form  of  a  uniform  coating 
or  film  over  the  cylinder  walls,  valve  seats,  and  piston  rod,  and 
lubricates  these  surfaces  in  a  most  economical  manner. 

Mechanically  operated  lubricators  have  several  advantages 
over  the  hydrostatic  lubricators.  The  oil  is  fed  only  when  the 
engine  is  running,  as  the  lubricator  pump  is  operated  from  the 
engine  and  starts  and  stops  with  the  engine.  This  is  more 
positive  and  reliable  and  a  more  uniform  feed  is  maintained. 

Certain  types  of  mechanical  lubricators,  however,  are  not  suit- 
able for  compound  condensing  engines  operating  on  a  light  load, 
unless  the  check  valves  on  the  discharge  side  of  the  pump  are 
spring  loaded,  as  a  vacuum  acting  on  the  oil  pipe  with  atmos- 
pheric pressure  on  the  reservoir  will  syphon  the  oil  from  the 
reservoir. 

Lubrication  of  Valves.  Slide  Valve. — As  a  slide  valve  is  posi- 
tively operated,  it  can  theoretically  be  operated  at  any  speed; 
but  in  actual  service,  due  to  its  unbalanced  construction,  it  is  not 
operated  at  as  high  speed  as  the  piston  valve. 

The  flat  surface  of  the  slide  valve  which  rubs  against  the  valve 
seat  is  difficult  to  lubricate,  particularly  when  the  slide  valve  is 
large.  In  some  extreme  cases,  oil  grooves  are  cut  in  the  valve  or 
in  the  valve  seat  to  assist  in  spreading  the  oil  all  over  the  fric- 
tional  surfaces. 

The  use  of  the  slide  valve  is  restricted  to  a  maximum  steam 
pressure  of  about  120  Ib.  or  a  maximum  steam  temperature  of 
about  450°  F.  This  is  due  to  the  large,  flat,  frictional  surfaces 
of  the  slide  valve  and  its  seat,  and  the  difficulty  of  introducing  the 
oil  thoroughly  between  them,  owing  to  the  great  pressure  on  the 
valve.  Excessive  steam  pressure  prevents  the  formation  and 
maintenance  of  the  oil  film,  resulting  in  metallic  contact  of  the 
rubbing  surfaces,  with  excessive  friction  and  wear.  Excessive 
steam  temperature  will  result  in  unequal  expansion  and  distor- 
tion of  the  valve  and  valve  seat.  Steam  will  leak  past  the  valve 
seat,  causing  cutting  of  the  surfaces.  As  the  oil  film  is  thinned 
out,  due  to  the  high  temperature,  it  will  be  unable  to  resist  the 
pressure  between  the  frictional  surfaces,  and  metallic  contact  and 
wear  will  follow. 


LUBRICATION  267 

Upon  removing  the  cover  from  the  steam  chest  for  inspection, 
excessive  friction  of  the  slide  valve  is  always  indicated  by  a  dry- 
ness  of  the  rubbing  surfaces,  which  will  show  wear  and  bright 
streaks  of  cutting.  Wear  on  the  valve  seat  will  be  reduced  if  the 
cast  iron  of  the  valve  seat  is  slightly  harder  than  that  of  the  valve. 

Improper  lubrication  results  in  abrasion  and  cutting;  excessive 
leakage  of  steam  takes  place  and  wipes  away  the  lubricating  oil 
from  the  valve  seat,  making  necessary  an  increased  consumption 
of  oil.  Friction  of  the  slide  valve  often  produces  groaning  during 
operation,  and  the  excessive  resistance  in  moving  the  valve  causes 
the  eccentric  rod  to  vibrate.  With  efficient  lubrication  the  valve 
operates  without  noise;  the  eccentric  rod  works  smoothly;  and, 
when  inspected,  the  friction  surfaces  of  the  valve  and  seat  have  a 
polished,  dull,  glossy  appearance. 

Experience  has  shown  that  when  the  oil  is  introduced  into  the 
steam  line  and  atomized,  it  is  most  thoroughly  distributed.  In 
many  cases,  atomizing  the  oil  has  furnished  the  lubrication  which 
has  overcome  groaning  and  other  troubles  with  slide  valves  where 
direct  methods  of  application  have  given  insufficient  results. 

Corliss  Valves. — Although  the  Corliss  valves  have  a  rotating 
motion  and  the  rubbing  surfaces  are  cylindrical,  conditions  of 
steam  pressure  and  temperature  affect  their  lubrication  in  the 
same  manner  as  with  slide  valves.  There  is  this  difference,  how- 
ever, that  insufficient  lubrication  will  cause  the  sluggish  closing 
of  the  admission  valves,  since  they  are  riot  connected  directly  to 
the  operating  mechanism  during  the  closing  period.  Otherwise, 
excessive  friction  is  indicated  by  groaning  and  vibration  of  the 
valve  and  operating  mechanism,  as  in  the  case  of  slide  valves. 

Corliss  valves  should  be  lubricated  by  introducing  the  oil  into 
the  steam  line  and  atomizing  it,  as  it  will  then  be  quickly  carried 
to  the  rubbing  surfaces.  Introducing  the  oil  directly  over  the 
valves  is  wasteful  or  inefficient  or  both. 

Piston  Valves. — On  account  of  the  symmetrical  shape  of  the 
piston  valve  and  sleeve  there  is  but  little  pressure  between  them. 
This  shape  also  permits  them  to  expand  uniformly  under  high 
steam  temperature.  This  type  of  valve  is,  therefore,  well 
adapted  to  running  at  very  high  speeds.  The  large  surface 
moved  over  by  the  piston  valve  at  high  speeds  demands  effective 
lubrication,  which  is  best  secured  by  the  atomization  method. 

Poppet  Valves. — Poppet  valves  have  no  sliding  motion  and, 
therefore,  do  not  require  lubrication,  but  the  valve  stems  which 


268  STEAM  ENGINES 

move  up  and  down  in  guides  require  a  small  amount  of  lubricant. 
External  lubrication  of  the  valve  stems  is  likely  to  cause  them  to 
stick,  owing  to  the  fact  that  the  clearance  between  guide  and 
valve  stem  is  very  small.  For  this  reason,  atomizing  the  oil  and 
using  it  sparingly  will  give  best  results. 

Piston  and  Cylinders. — In  the  lubrication  of  steam-engine 
cylinders  we  have  to  consider  two  types  of  engines,  vertical  and 
horizontal.  In  vertical  engines  the  pressure  between  the  piston 
and  cylinder  is  moderate,  being  due  almost  entirely  to  the  spring- 
ing action  of  the  rings.  For  this  reason  less  oil  is  required  for 
lubrication  than  with  horizontal  engines.  In  horizontal  engines, 
the  lower  part  of  the  cylinder  carries  the  weight  of  the  piston  in 
addition  to  the  pressure  of  the  rings.  Some  large  horizontal 
engines  have  tail  rods  which,  together  with  the  piston  rod,  carry 
the  major  portion  of  the  weight  of  the  piston.  The  duty  of  the 
piston  rings,  then,  is  simply  to  prevent  leakage  of  steam.  The 
pressure  of  the  piston  has  an  important  bearing  on  cylinder 
lubrication  because  the  oil  film  is  at  a  high  temperature  and 
excessive  pressure  may  easily  destroy  it,  resulting  in  friction  and 
wear. 

The  inside  of  a  properly  lubricated  steam-engine  cylinder  will 
have  a  dull  appearance  due  to  the  presence  of  a  film  of  oil.  The 
presence  of  the  oil  can  be  detected  by  wiping  the  surface  with  a 
piece  of  white  paper,  the  stain  left  by  the  oil  having  a  brownish 
color.  If  the  stain  is  black  it  is  an  indication  that  the  oil  is 
being  carbonized.  When  the  film  of  oil  has  been  wiped  away  the 
surface  underneath  should  appear  dull  and  glossy.  If  wear  has 
taken  place  it  will  be  indicated  by  the  surface  being  bright  and 
there  will  usually  be  streaks  or  scratches  also. 

Piston  and  Valve  Rods. — Piston  and  valve  rods  are  always  pro- 
vided with  stuffing  boxes  to  prevent  leakage  of  steam  out  of  the 
cylinder  or  valve  chest,  and,  in  the  case  of  low-pressure  cylinders, 
to  prevent  the  leakage  of  air  into  the  cylinders  when  the  engine  is 
operated  condensing. 

Stuffing  boxes  are  packed  with  either  soft  or  metallic  packing. 
Full  and  efficient  lubrication  of  the  packing  is  essential  as  a 
perfect  seal  can  only  be  obtained  by  the  presence  of  a  complete  oil 
film  on  the  rods. 

Soft  packings  are  used  only  under  moderate  steam  conditions. 
The  friction  between  the  packing  and  the  rod  is  always  com- 
paratively high,  and  if  the  packing  is  screwed  up  too  tight,  it 


LUBRICATION  269 

causes  grooving  or  scoring  of  the  rod,  and  it  is  then  difficult  to 
prevent  leakage  of  steam. 

In  reversing  engines,  the*  engine  is  reversed  by  changing  the 
movement  of  the  valves  with  relation  to  the  position  of  the  pis- 
tons. This  is  done  by  hand  with  small  engines  and  by  a  special 
reversing  engine  in  the  case  of  large  engines.  In  either  case  the 
pull  required  to  reverse  the  engine  depends  upon  the  friction  of 
the  valves  moving  over  their  seats  and  by  the  friction  of  the  rod 
moving  through  the  stuffing  box.  Where  the  valve  rods  have 
been  lubricated  externally  by  the  direct  method,  which  is  waste- 
ful and  inefficient,  a  change  to  the  atomization  method  effects  a 
marked  improvement.  The  valve  rod  receives  lubrication 
when  inside  the  valve  chest  and  furnishes  efficient  lubrication  to 
the  packing.  As  a  result,  external  lubrication  of  the  valve  rods 
can  be  dispensed  with. 

Metallic  packing  is  much  superior  to  soft  packing  and  it  should 
be  used  where  the  steam  temperature  is  high.  The  friction  of 
metallic  packing  is  less  than  with  soft  packing  as  it  exerts  only  a 
slight  pressure,  and  there  is  less  danger  of  scoring  the  piston  rod 
with  it.  With  highly  superheated  steam  it  is  usually  necessary 
to  supply  a  small  amount  of  direct  lubrication  to  metallic  packing 
in  addition  to  that  supplied  by  the  atomization  method,  but  oil 
used  in  this  way  should  be  used  sparingly  as  an  excess  of  oil 
may  clog  the  packing  or  become  carbonized.  With  only  moderate 
superheat,  direct  lubrication  can  usually  be  dispensed  with. 

Influence  of  Operating  Conditions. — Stationary  steam  engines 
usually  operate  at  constant  speed,  the  changes  of  load  being  taken 
care  of  by  changing  the  volume  of  steam  admitted  to  the  cylinder 
or  by  changing  its  pressure.  Either  will  reduce  the  velocity  of 
steam  flow  through  the  steam  main  at  reduced  loads.  As  this 
will  affect  the  atomization  of  cylinder  oil  it  should  be  considered 
in  selecting  the  oil.  With  full  velocity  of  the  steam,  heavy  oils 
will  be  readily  atomized,  but  a  low  velocity  of  steam  requires 
a  lighter  oil  in  order  to  secure  thorough  atomization  and  good 
distribution. 

The  speed  ot  the  engine  also  affects  the  lubrication.  High- 
speed engines,  making  short  quick  strokes,  take  in  only  a  small 
volume  of  steam  at  each  stroke,  so  that  only  a  light-bodied, 
quick-acting  oil  will  give  efficient  lubrication. 

If  the  supply  of  steam  to  the  engine  is  wet  it  makes  efficient 
lubrication  more  difficult  because  the  wet  steam  has  a  tendency  to 


270  STEAM  ENGINES 

wash  the  film  of  oil  off  the  surfaces  of  the  cylinder.  Oil  to  be  used 
with  wet  steam  should  have  great  endurance  and  must  be  of  such 
quality  that  it  will  readily  combine  with  moisture  and  cling  to  the 
cylinder  walls. 

With  compound  and  triple-expansion  engines,  even  when 
the  supply  of  steam  is  dry,  the  drop  of  pressure  causes  condensa- 
tion so  that  the  supply  of  steam  to  the  following  cylinders  will 
be  wet.  It  is,  therefore,  necessary  sometimes  to  use  one  grade 
of  cylinder  oil  for  the  high-pressure  cylinder  and  a  different 
grade  for  the  low-pressure  cylinder  in  order  to  secure  efficient 
lubrication. 

The  use  of  highly  superheated  steam  requires  the  highest 
quality  of  oil  as  the  friction  and  high  temperature  will  carbonize 
and  decompose  poor  quality  oils.  If  the  engine  operates  under 
fairly  heavy  load  a  heavier-bodied  oil  may  be  used  than  when  the 
engine  operates  at  comparatively  light  loads.  Under  the  latter 
condition  a  medium-bodied  oil  is  best. 

Low-grade  mineral  and  compounded  cylinder  oils  are  difficult 
to  separate  from  the  exhaust  steam.  The  best  grades  of  mineral 
oils  separate  more  easily  from  exhaust  steam  and  feed  water  than 
do  compounded  oils;  it  will,  however,  be  found  that  more  oil  will 
be  required  to  give  effective  lubrication  when  using  a  mineral  oil 
than  when  using  a  compounded  oil. 


CHAPTER    XX 
STEAM  TURBINES 

General  Principles. — The  principles  of  the  steam  turbine  and 
the  operation  of  steam  in  it  are  entirely  different  from  the  princi- 
ples of  the  steam  engine  and  the  operation  of  steam  in  it.  In  a 
steam  engine  some  of  the  moving  parts  have  a  reciprocating 
motion  and  the  action  of  the  steam  is  intermittent,  while  a  steam 
turbine  is  an  apparatus  in  which  the  moving  parts  have  only  a 
rotating  motion  and  the  steam  which  passes  through  it  acts  in 
such  manner  as  to  produce  a  constant  angular  velocity. 

In  order  to  illustrate  in  a  homely  way  the  difference  between 
the  action  of  a  steam  engine  and  that  of  a  steam  turbine,  consider 
a  large  wheel  supported  in  a  horizontal  plane  on  a  vertical  shaft. 
The  wheel  may  be  rotated  by  grasping  the  rim  and  walking  con- 
tinuously around  the  shaft.  In  a  similar  manner  the  pressure 
of  the  steam  acts  upon  the  piston  of  a  steam  engine  and  pushes  it 
forward.  The  wheel  may  also  be  turned  by  standing  in  one  spot 
and  grasping  the  rim  and  moving  it  with  first  one  hand  and  then 
the  other  in  a  similar  manner  to  opening  or  closing  a  large  valve 
by  hand.  In  this  case  the  wheel  is  not  turned  by  exerting  a 
pressure  at  one  point  on  its  rim,  as  in  the  previous  case,  but  by 
exerting  a  pressure  at  first  one  point  on  the  rim  and  then  another 
so  that  all  points  on  the  rim  are  used  successively.  This  is  the 
manner  in  which  steam  acts  in  a  steam  turbine. 

In  order  to  make  the  wheel  turn  while  standing  in  one  spot  we 
must  have  a  firm  place  to  stand  on  and  a  good  grip  on  the  floor  in 
order  to  be  able  to  exert  the  required  pressure  on  the  wheel.  This 
means  that  in  every  turbine  there  must  be  certain  stationary 
parts  which  are  firmly  fixed  to  the  casing  in  order  that  the  steam 
may  have  a  good  grip  on  the  moving  or  revolving  parts  which  are 
fastened  to  the  power-transmitting  shaft. 

Although  we  can  readily  understand  how  we  may  turn  the 
wheel  by  standing  in  one  point,  it  is  not  so  easy  to  understand 
how  steam,  which  is  a  flexible  medium,  can  grip  the  wheel  of  a 
turbine  in  such  manner  as  to  turn  it. 

28  271 


272 


STEAM  ENGINES 


There  are  two  methods  by  which  this  may  be  done  and  these 
will  be  illustrated  by  the  following  examples  in  which  water  is 
used  instead  of  steam,  remembering  that  the  actions  taking  place 
with  steam  will  be  the  same  as  those  taking  place  with  water. 

If  a  hose  nozzle  be  directed  against  a  board  arranged  as  shown 
in  Fig.  164,  the  water  flowing  from  the  nozzle  will  exert  a  pressure 
on  the  board  as  indicated  by  the  scale  at  the  left.  The  pressure 
exerted  upon  the  board  will  depend  upon  the  velocity  of  the  water 
and  upon  the  angle  at  which  the  water  strikes  the  board.  If  the 
stream  of  water  is  directed  almost  parallel  with  the  board  it  will 
exert  only  a  small  pressure  but  if  it  is  directed  so  that  it  strikes 


FIG.  164. 

the  board  almost  perpendicular  to  its  surface  it  will  exert  a 
much  larger  pressure. 

With  the  arrangement  of  the  nozzle  and  board  shown  in  Fig. 
164  it  will  be  appreciated  that  the  stream  of  water  will  break  and 
cause  it  to  splash.  This  reduces  the  pressure  which  the  stream 
of  water  can  exert.  If,  however,  the  board  is  curved,  as  shown 
in  Fig.  165,  the  water  will  pass  over  its  surface  in  a  smooth  stream 
without  splashing  and  at  the  same  time  the  nozzle  may  be  directed 
so  as  to  produce  the  maximum  pressure.  The  pressure  exerted 
by  the  stream  of  water  is  due  entirely  to  changing  the  direction 
in  which  the  stream  is  flowing.  The  amount  or  intensity  of  the 
pressure  will  depend  upon  the  velocity  of  the  stream,  upon  the 
weight  of  the  water  (or  steam)  and  upon  the  angle  through 
which  it  is  turned. 


STEAM  TURBINES 


273 


The  velocity  of  the  stream  when  it  leaves  the  board  will  be 
practically  the  same  as  its .  entering  velocity,  the  only  loss  of 
velocity  being  that  due  to', the  friction  of  the  water  passing  over 
the  surface  of  the  board;  this  is  small  if  the  board  is  smooth. 
Moreover,  if  the  water  leaves  the  board  at  the  same  angle  at 
which  it  strikes,  it,  that  is,  if  the  curvature  of  the  board  is 
uniform,  the  pressure  will  be  directed  along  the  axis  of  the  board 
as  indicated  by  the  arrow,  and  there  will  be  no  side  thrust. 

The  above  discussion  has  shown  how  a  stream  of  water  directed 
along  a  curved  surface  can  produce  a  pressure,  but  in  the  case 
considered,  no  work  was  done  because  the  curved  board  does  not 
move.  In  order  for  work  to  be  done,  a  force  must  act  or  move 


FIG.  165. 

through  a  distance.  The  product  of  the  force  and  the  distance 
will  then  be  the  foot-pounds  of  work  done.  If  the  curved  board 
is  fastened  to  the  rim  of  a  wheel,  as  shown  in  Fig.  165,  and  the 
wheel  is  fastened  to  an  axle,  then  the  pressure  on  the  board  due  to 
the  stream  of  water  will  cause  the  wheel  to  turn  and  to  do  work. 
If,  instead  of  only  one  of  the  curved  boards,  or  blades  as  we  will 
now  call  them,  a  number  of  blades  are  placed  around  the  circum- 
ference of  the  wheel  so  that  as  soori  as  one  blade  has  moved  out 
of  the  stream  of  water  another  will  move  into  it,  a  continual 
pressure  will  be  exerted  and  the  wheel  will  revolve  uniformly. 
Also,  instead  of  only  one  nozzle  and  stream  of  water,  there  may 
be  several  nozzles  spaced  at  intervals  around  the  circumference 


274  STEAM  ENGINES 

so  that  instead  of  the  vanes  receiving  pressure  from  one  nozzle 
they  may  receive  pressure  from  several  nozzles  and  thus  increase 
the  total  pressure  acting  upon  the  rim  of  the  wheel.  This  con- 
stitutes a  turbine  similar  to  one  type  which  is  in  common  use, 
a  type  known  as  the  impulse  turbine. 

In  discussing  the  action  of  the  stream  of  water  on  the  curved 
vanes  it  has  been  assumed  that  the  vanes  were  standing  still.  It 
remains  now  to  discuss  the  effects  produced  by  the  movement  of 
the  vanes,  because  this  is  the  condition  that  will  exist  when  the 
turbine  is  running.  This  is  important  because  the  pressure  on 
the  vane  depends  upon  the  velocity  with  which  the  water  is 
moving  over  the  surface  of  the  vane  and  this  velocity  is  affected 
very  much  by  the  movement  of  the  vane.  In  other  words,  the 


FIG.   166. 

velocity  of  the  stream  relative  to  the  vanes  is  entirely  different 
from  the  absolute  velocity  of  the  stream. 

To  illustrate  this  and  to  show  how  the  velocity  of  the  stream 
relative  to  the  vane,  when  the  vane  is  moving,  may  be  determined, 
consider  the  case  of  a  person  walking  across  the  floor  of  a  railway 
coach  when  the  coach  is  moving.  Referring  to  Fig.  166,  suppose 
the  train  is  moving  forward  with  a  velocity  represented  by  the 
length  of  the  line  A B  and  that  the  direction  of  this  line  also  repre- 
sents the  direction  in  which  the  point  A  on  the  floor  of  the  coach  is 
moving.  Now  suppose  a  person  standing  at  A  walks  across  the 
coach  in  the  direction  AD  and  walks  a  distance,  measured  on  the 
floor  of  the  coach,  represented  by  the  line  AD,  in  the  same  length 
of  time  that  the  train 'covers  the  distance  AB.  Then  the  actual 
motion  of  the  person  with  respect  to  the  ground  is  represented  in 
direction  and  length  by  the  line  AC  which  is  the  diagonal  of  the 
figure  A  BCD.  That  is,  the  line  AB  represents  in  length  and 


STEAM  TURBINES 


275 


direction  the  absolute  velocity  of  the  train  with  respect  to  the 
ground,  the  line  AC  represents  the  absolute  velocity  of  the  person 
with  respect  to  the  ground^  and  the  line  AD  represents  the  ve- 
locity of  the  person  relative  to  the  train. 

Applying  the  same  kind  of  diagram  to  the  case  of  a  stream  of 
steam  passing  over  the  surface  of  a  vane  of  a  steam  turbine  we 
would  have  a  figure  similar  to  the  one  shown  in  Fig.  167.  In  this 
diagram  the  curved  line  AG  represents  the  curved  surface  of  the 
vane.  The  line  AD  represents  the  velocity  of  the  vane  as  the 
wheel  turns,  the  line  AE  represents  the  velocity  of  the  steam  as  it 
leaves  the  nozzle  and  strikes  the  vane,  and  the  line  AF  represents 


FIG.  167. 

the*  velocity  of  the  steam  with  respect  to  the  surface  of  the  vane. 
That  is,  instead  of  the  entire  velocity  of  the  steam  AE  being 
effective  in  creating  pressure  against  the  vane,  there  is  only  the 
velocity  AF  to  produce  this  pressure. 

Consider  next  what  happens  to  the  velocity  as  the  stream  leaves 
the  vane.  This  is  shown  by  a  similar  velocity  diagram  drawn 
about  G,  the  point  at  which  the  steam  leaves  the  vane.  As  the 
steam  passes  over  the  surface  of  the  vane,  its  direction  is  changed 
so  that  if  the  vane  were  standing  still  it  would  leave  in  the  direc- 
tion GH ,  tangent  to  the  vane.  The  line  GH  therefore  represents 
the  velocity  of  the  steam  with  respect  to  the  vane.  The  line  GH 
will  have  practically  the  same  length  as  the  line  AF  because  the 
relative  velocity  of  the  steam  with  respect  to  the  vane  will  be 


276  STEAM  ENGINES 

practically  the  same  at  the  outlet  end  of  the  vane  as  at  the  inlet, 
the  only  loss  in  this  velocity  being  that  due  to  the  friction  of 
the  steam  on  the  surface  of  the  vane.  This  is  small  because  the 
vanes  are  made  as  smooth  as  possible.  The  line  GJ  represents 
by  its  length  and  direction  the  velocity  with  which  the  vane  is 
moving,  and  it  is,  of  course,  the  same  as  the  line  AD.  The 
diagonal  of  the  diagram,  or  the  line  GK  therefore  represents  by  its 
length  the  absolute  velocity  with  which  the  steam  leaves  the 
moving  vane  and  its  direction  shows  the  direction  in  which  the 
steam  leaves  the  vane.  That  is,  the  steam  enters  the  moving 
vane  with  an  absolute  velocity  of  AE  and  leaves  it  with  an  abso- 
lute velocity  of  GK. 

Having  considered  the  velocities  involved  in  the  operation  of  a 
steam  turbine,  we  are  in  a  position  to  study  the  amount  of  energy 
obtained  from  these  velocities.  If  we  determine  the  amount  of 
energy  in  the  steam  entering  the  vane  and  subtract  from  this 
the  amount  of  energy  in  the  steam  as  it  leaves  the  vane,  the  differ- 
ence must  evidently  be  the  amount  of  energy  given  to  the  vane. 
This  represents  the  energy  developed  by  the  turbine  wheel,  if  we 
neglect  the  small  loss  of  energy  that  takes  place  in  the  vanes. 

If  a  weight  of  G  pounds  of  working  fluid  (steam  or  water)  enters 
the  vane  per  second  with  a  velocity  of  v\  feet  per  second,  its  ki- 
netic energy  or  energy  of  motion  is 

GV 
20 

in  which  formula  g  represents  the  acceleration  due  to  gravity, 
32.2  ft.  per  sec.  per  sec. 

Also  the  kinetic  energy  of  the  working  fluid  as  it  leaves  the  vane 
is 

"20" 

The  difference  between  these  two  quantities  is  the  energy  given  to 
the  wheel,  W,  and  the  loss  L,  or 

/3,,  2          /QL.,.2 

W  +L 


20         20 
or 


This  equation  means  that  we  get  out  of  the  steam  more  useful 
energy  W ,  the  greater  G  is,  the  smaller  the  losses,  and  especially 


STEAM  TURBINES  277 

the  larger  viz  —  vzz  is.  With  a  given  inlet  velocity  the  expression 
t>i2  —  z;22  will  be  larger,  the  smaller  the  absolute  outlet  velocity  t>2 
is  as  compared  with  the  absolute  inlet  velocity  v\.  It  is  evident 
from  Fig.  167  that  the  absolute  outlet  velocity  vz  will  be  smallest 
when  the  line  GK  is  at  right  angles  to  GJ;  that  is,  when  the  steam 
leaves  the  vanes  at  right  angles  to  the  direction  in  which  the 
vanes  are  moving.  This,  therefore,  is  one  of  the  conditions  for 
maximum  efficiency  in  a  steam  turbine  and  the  designer  chooses 
the  angles  and  velocities  so  as  to  secure  this  result  as  nearly  as 
possible. 

It  can  be  shown  mathematically  that  another  condition  for 
maximum  efficiency  is  that  the  circumferential  speed  of  the 
wheel,  c,  should  be  approximately  one-half  of  the  absolute  inlet 
velocity  of  the  steam,  vi,  or 

9t 

c=  2 

It  will  thus  be  seen  that  the  inlet  velocity  is  a  very  important 
quantity.  Let  us  see,  therefore,  what  we  may  expect  this  velocity 
to  be. 

In  water  turbines  the  absolute  inlet  velocity  of  a  simple  impulse 
turbine  depends  merely  upon  the  height  or  head  of  the  column  of 
water  above  the  level  of  the  turbine  wheel.  Suppose  this  head 
to  be  "h"  feet,  then  the  theoretical  absolute  inlet  velocity  "v"  is 

v  =  \/2gh 

This  means,  for  instance,  that  a  particle  of  water  flowing  down 
from  a  height  of  100  feet,  reaches  a  velocity  of 

v  =  \/2  X  32.2  X  100 
=  80  feet  per  second,  approximately. 

Taking  one  pound  of  water,  in  a  height  of  100  feet  above  turbine 

level,  we  may  say  that  the  energy  stored  up  in  the  pound  of  water 

is  100  foot-pounds.    In  falling  through  the  height  of  100  feet  this 

stored-up  energy  is  changed  into  energy  of  motion,  so  that  the 

.  energy  of  motion  is 

Gv2        1  X  802 

E  =  -TT—  =  o  v y  oo  o  =  1°°  foot-pounds,  approximately.     In 
Zg        *  X  oz.z 

other  words,  the  stored  up  energy  at  the  top  of  the  column  of 
water  is  equal  to  the  energy  of  motion  at  the  foot  of  the  column 
(100  foot-pounds)  100  feet  below,  in  accordance  with  the 
law  of  constant  energy.  How  is  this  with  a  steam  turbine.  It  is 


278  STEAM  ENGINES 

exactly  the  same,  but  until  recently  no  one  thought  of  comparing 
steam  under  pressure  in  a  boiler  with  water  under  a  very  high 
head,  because  such  a  comparison  was  not  necessary  in  steam- 
engine  practice. 

Suppose  we  consider  one  pound  of  steam  in  a  boiler  under  a 
pressure  of  "p"  pounds  per  square  inch.  What  is  the  amount 
of  energy  stored  up  in  this  pound  of  steam  expressed  in  foot- 
pounds? If  we  allow  this  pound  of  steam  to  expand  to  its 
absolute  pressure  "pi,"  with  a  volume  of  V\  (whereas  the  .volume 
at  the  pressure  "p"  may  -be  V)  then  the  amount  of  stored  up 
energy  L  in  foot-pounds  is 

vV     f          iV 

:  ;...*- A  Mr, 

and  during  this  expansion  the  volume  increases  according  to  the 
law  pVn  equals  a  constant  quantity,  n  being  equal  to  1.135. 
Under  average  boiler  and  condenser  conditions  this  L  is  234,000 
foot-pounds.  (Steam  pressure  150  pounds,  vacuum  28  inches.) 
To  understand  clearly  this  figure,  234,000  foot-pounds,  we 
must  bear  in  mind  that  the  amount  of  stored  up  energy  in  one 
pound  of  water  with  a  head  equal  to  that  of  Niagara  Falls  is 
only  150  foot-pounds.  The  head  of  one  pound  of  steam  in  a 
boiler  under  150  pounds  pressure  against  28  inches  of  vacuum 

234  000 
is  therefore  — db^ —  =  1560  times  as  high  as  that  of  Niagara  Falls, 


and  for  such  enormous  heads  steam  turbines  are  designed. 
The  equation 

v=  \/2gh 

gives  us  some  idea  of  the  velocity  this  working  fluid  reaches  when 
it  flows  down  this  head 

v  =  A/2  X  32.2  X  234,000 
=  3880  feet  per  second  approximately. 

This  means  that  the  absolute  inlet  velocity  of  a  working  fluid 
expanding  from  150  pounds  per  square  inch  down  to  28  inches  . 
vacuum  would  be  3880  feet  per  second.    If  the  circumferential 
velocity  of  the  running  wheel  is  one-half  of  this  it  would  be 

3880 
c  =  —n—  =  1940  feet  per  second 

z 

or  116,400  feet  per  minute 
or  1318  miles  per  hour 


STEAM  TURBINES 


279 


Up  to  the  present  time  no  material,  not  even  the  best  nickel 
steel,  can  withstand  the  enormous  strains  due  to  such  high  cen- 
trifugal force. 

Unless  we  allow  a  considerably  greater  ratio  of  inlet  velocity 
to  circumferential  velocity  than  two  to  one  we  are  unable  to 
build  a  turbine  in  this  way  with  only  one  wheel.  As  soon,  how- 
ever, as  we  increase  this  ratio,  we  have  to  expect  an  uneconom- 
ically  working  turbine.  Therefore,  as  it  is  absolutely  necessary  to 
build  a  turbine  so  it  will  be  as  economical  as  possible,  we  have  to 
seek  other  ways  to  reach  practical  limits  of  circumferential 
velocity.  The  best  way  to  do  this  is  shown  in  the  practice  of 
water  turbines,  as  for  instance,  in  Massachusetts  the  manufactur- 
ing industries  are  using  the  water  power  of  the  Connecticut  River 


HIGH  LEVEL. 


1 


TH/frVD  LEVEL 


fOURTH  LEVEL. 


VIE  W 

FIG.  168. 

in  such  a  way  that  they  lead  the  power  canal  in  loops  as  shown 
in  Fig.  168.  The  circles  here  represent  the  water  turbines. 
It  is  plain  that  the  first  row  of  turbines  uses  the  head  between  high 
level  and  second  level;  the  second  row  of  turbines  uses  the  head 
between  second  level  and  third  level,  and  so  on.  In  short,  the 
whole  head  between  high  level  and  low  level  is  split  into  a  num- 
ber of  fractional  heads  between  intermediate  levels. 

This  arrangement  of  water  turbines  indicates  plainly  the  way 
to  use  the  enormous  heads  of  steam  turbines  with  practical  means, 
and  yet  follow  the  fundamental  laws  as  given  above.  In  other 
words,  in  order  to  get  an  economically  working  steam  turbine 
with  moderate  circumferential  speeds  of  the  running  wheel, 
we  must  use  a  number  of  running  wheels,  all  mounted  upon  one 
shaft,  each  of  these  running  wheels  to  be  driven  by  steam  flowing 
through  a  corresponding  number  of  stationary  blades  where 


280 


STEAM  ENGINES 


the  steam  partly  expands  and  accelerates  to  a  velocity  approxi- 
mately twice  the  circumferential  velocity. 

One  of  the  first  steam  turbines  that  came  into  extensive  use 
in  this  country  makes  use  of  the  first  principle  stated  above; 
that  is,  it  has  only  one  running  wheel  and  develops  all  of  its 
power  in  this  single  wheel.  This  turbine  is  called  the  De  Laval 
turbine.  It  is  made  in  small  and  medium  sizes  up  to  about 
350  horsepower  but  it  is  not  well  adapted  for  large  units.  Hav- 
ing only  one  running  wheel  in  which  the  energy  of  the  steam  is 
utilized,  this  wheel  must  necessarily  run  at  a  very  high  circum- 
ferential velocity  (about  1200  feet  per  second  or  72,000  feet  per 
minute) . 


FIG.  169. 


An  illustration  of  a  De  Laval  turbine  wheel  and  nozzles  is 
shown  in  Fig.  169.  This  illustration  shows  the  casing  of  the 
turbine  removed  so  that  the  construction  of  the  nozzles  and 
wheel  may  be  seen  more  clearly.  Instead  of  only  one  nozzle,  it 
will  be  seen  that  four  nozzles  are  used  to  direct  the  steam  into  the 
vanes,  as  this  brings  into  operation  a  larger  portion  of  the  cir- 
cumference of  the  wheel  and  permits  four  times  as  much  power 
to  be  developed  as  if  only  one  nozzle  was  used. 

The  shape  of  the  nozzles  is  clearly  shown  in  Fig.  169.  This 
shape  is  such  that  the  steam  is  expanded  in  the  nozzle  and  its 
pressure  energy  changed  into  velocity  energy,  at  the  same  time 
causing  very  little  loss  of  energy  in  passing  through  the  nozzle. 
When  the  steam  enters  the  nozzle,  it  has  a  high  pressure  and 
small  volume.  This  is  changed  in  the  nozzle  so  that  as  the  steam 


STEAM  TURBINES 


281 


leaves  it,  it  has  only  a  low  pressure  but  occupies  a  large  volume, 
and  consequently  it  leaves*  the  nozzle  and  enters  the  vanes  with 
a  very  high  velocity.  The  v&nes  are  spaced  uniformly  around 
the  circumference  of  the  wheel  and  they  are,  of  course,  deep 
enough  to  accommodate  the  required  volume  of  steam. 

A  study  of  the  diagrammatic  sketch  shown  in  Fig.  170  will 
give  a  good  idea  of  the  changes  of  pressure  and  velocity  that  take 
place  in  a  De  Laval  turbine.  The  upper  part  of  the  sketch 


BOILER 
PRESSURE 


EXHAUST  F>f?ESSURE 


CONDENSER  Of? 

'EXHAUST  /=>/?£:SSURE 


FIG.  170. 

shows  an  elevation  of  the  turbine  shaft,  the  nozzle,  and  one-half 
of  the  wheel.  Just  below  this  is  shown  a  plan  view  of  the  wheel 
and  nozzle.  At  the  bottom  are  shown  lines  representing  the 
steam  pressure  and  velocity  in  different  parts  of  the  turbine, 
the  height  of  these  lines  above  the  base  line  being  proportional 
to  the  pressure  and  velocity.  It  will  be  observed  that  the  steam 
pressure  decreases  within  the  nozzle  from  boiler  pressure  to 
exhaust  or  condenser  pressure,  that  the  running  wheel  revolves 
in  this  low  pressure,  having  the  least  possible  resistance,  and 


282  STEAM  ENGINES 

further,  that  the  steam  velocity  reaches  its  maximum  at  the 
outlet  of  the  nozzle  and  decreases  within  the  vanes  of  the  running 
wheel,  thus  transmitting  the  energy  of  the  steam  to  the  turbine 
shaft,  and  that  it  leaves  the  vanes  with  a  certain  lost  or  unused 
velocity.  This  unused  velocity  is  necessary  in  any  turbine  to 
carry  the  exhaust  steam  away  from  the  running  wheel. 
The  above  diagram  shows  further  that  the  pressure  is  the 
same  on  both  sides  of  the  running  wheel,  and  therefore,  the 
axial  thrust  due  to  steam  pressure  is  theoretically  zero  in  this 
turbine. 

That  class  of  steam  turbines  which,  instead  of  utilizing  the 
energy  of  the  steam  in  a  single  running  wheel,  divide  it  among 
several  running  wheels  and  thereby  permit  a  much  lower  cir- 
cumferential velocity,  is  well  represented  by  the  Curtis  steam 
turbine,  which  is  used  very  extensively  in  this  country.  These 
turbines  are  made  in  all  sizes  from  the  smallest  to  the  largest 
and  on  account  of  their  smaller  number  of  revolutions  per  min- 
ute they  can  be  connected  directly  to  electrical  machinery  with- 
out it  being  necessary  to  use  a  set  of  reduction  gears  between 
the  turbine  shaft  and  the  shaft  of  the  driven  machinery. 

A  diagrammatic  sketch  of  the  Curtis  turbine  arranged  similar 
to  the  one  for  the  De  Laval  turbine,  is  shown  in  Fig.  171.  It 
will  be  observed  that  the  running  wheels  in  this  turbine  are 
divided  into  two  sets.  Each  set  consists  of  two  running  wheels, 
marked  R,  and  a  stationary  wheel  or  set  of  vanes,  marked  S, 
between  them.  Steam  is  directed  against  the  first  set  of  running 
vanes  by  nozzles  similar  in  shape  to  those  used  in  the  De  Laval 
turbine.  The  stationary  vanes  between  the  two  running  wheels 
are  merely  for  the  purpose  of  changing  the  direction  of  the  steam 
so  that  the  steam  may  strike  each  set  of  running  vanes  in  the 
same  direction. 

In  the  first  nozzles  one  part  of  the  pressure  energy  is  converted 
into  velocity  energy.  With  the  velocity  corresponding  to  this 
partial  expansion  of  the  steam  the  first  running  wheel  is  impinged 
and  one  fraction  of  the  whole  steam  velocity  is  made  available 
in  this  wheel.  Leaving  this  wheel,  the  steam  is  led  through 
intermediate  or  stationary  vanes  to  the  next  running  wheel, 
where  the  remaining  part  of  the  velocity  due  to  the  first  expansion 
is  made  available. 

The  diagram  shows  the  decrease  of  the  pressure  from  boiler 
pressure  to  a  medium  pressure  in  the  first  nozzles,  the  steam 


STEAM  TURBINES 


283 


pressure  remaining  the  same  in  the  whole  first  part  of  the  turbine 
no  further  expansion  taking. place  in  it. 

After  having  left  the  la#t  running  wheel  of  the  first  part  of 
the  turbine,  the  steam  is  compelled  to  pass  through  nozzles  in 
which  it  expands  down  to  condenser  or  exhaust  pressure  thereby 
increasing  its  velocity  again  up  to  a  certain  maximum.  This 


FIG.  171. 

steam  jet  impinges  the  first  running  wheel  of  the  second  part 
of  the  turbine,  makes  one  fraction  of  its  velocity  available  in  it, 
leaves  the  wheel  with  a  certain  velocity,  is  led  with  this  velocity 
through  the  following  stationary  vanes  and  makes  the  rest  of 
the  velocity  available  in  the  following  running  wheel. 


284 


STEAM  ENGINES 


The  diagram  shows  the  changing  steam  pressure  and  steam 
velocity  very  plainly.  Of  course,  the  number  of  running  wheels 
and  the  number  of  sets  into  which  they  are  divided  depends  upon 
the  conditions  under  which  the  turbine  is  to  run.  In  the  above 
figure  we  merely  take  schematically  two  sets  of  wheels  with  two 
wheels  in  each  set  to  illustrate  the  principles  of  the  turbine. 


FIG.  172. 

The  intermediate  blades  in  each  stage  and  the  stationary 
blade  separating  one  stage  from  another  need  openings  only 
over  a  certain  part  of  the  circumference.  The  openings  in  the 
intermediate  vanes  increase  in  radial  height  according  to  the 
decreased  velocity  if  there  is  more  than  one  intermediate  row 
in  one  stage,  and  the  openings  of  the  stationary  blades  increase 


STEAM  TURBINES 


285 


n  radial  height  or  in  the  part  covering  the  circumference  ac- 
cording to  the  increasing  volume. 

In  the  Curtis  turbine,  as  in  the  De  Laval,  the  steam  pressure  is 
the  same  on  both  sides  of  e&ch  running  wheel,  hence  there  is  no 
axial  thrust  along  the  shaft. 


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FIG.  173. 


The  two  kinds  of  turbines  described  above  belong  to  the  general 
type  known  as  impulse  turbines.  Another  well-known  type  of 
turbine  operates  by  both  the  impulse  of  the  steam  against  the 
vanes  and  by  the  reaction  of  the  steam  as  it  leaves  the  vanes. 

An  ordinary  lawn  sprinkler  such  as  that  shown  in  Fig.  172  is  a 


286  STEAM  ENGINES 

good  illustration  of  the  force  of  reaction  exerted  by  a  stream  of 
flowing  fluid.  In  this  case  the  streams  of  water  flowing  from  the 
ends  of  the  arms  of  the  sprinkler  exert  a  backward  push  or 
"reaction"  which  is  sufficient  to  rotate  the  arms.  In  the  same 
way,  steam  flowing  at  high  velocity  from  a  properly  shaped 
nozzle  or  vane  will  produce  a  reaction.  It  is  this  reactive  force 
that  is  used  in  the  type  of  turbines  mentioned  above.  One  of 
the  best  known  of  this  type  is  the  Westinghouse-Parsons  turbine, 
which  is  illustrated  schematically  in  Fig.  173. 

No  nozzles  are  used  in  this  turbine,  but  the  rows  of  stationary 
vanes  take  the  place  of  nozzles  and  guide  the  steam  into  the 
adjacent  rows  of  running  vanes.  The  outlet  opening  between  the 
running  vanes,  being  smaller  than  the  inlet  opening,  the  steam 
is  compelled  to  expand  and  accelerate  within  these  running 
vanes  exerting  thus  a  back  pressure  upon  them  when  the  steam 
leaves.  In  this  turbine  there  are  therefore  two  ways  in  which 
the  steam  moves  one  vane.  First,  the  impact  at  the  inlet  and, 
second,  the  reaction  at  the  outlet.  On  account  of  the  larger 
angles  that  must  be  used  in  shaping  the  vanes  there  cannot 
be  made  available  in  each  row  such  a  large  fraction  of  energy 
as  can  be  made  available  by  using  pure  impact  and  considerably 
smaller  angles. 

The  steam  leaving  the  first  row  of  running  vanes  flows  through 
the  next  row  of  stationary  ones,  expands  there,  and  impinges 
with  the  new  velocity  the  second  row  of  running  vanes,  and  so  on, 
the  vanes  increasing  in  radial  height  in  conformity  with  the 
expansion  line  of  the  steam  and  the  diameter  increasing,  too, 
in  order  to  reach  higher  circumferential  velocities. 

The  schematical  diagram  shows  plainly  that  the  pressure  line  is 
nearly  one  continuous  curve  from  the  inlet  to  the  outlet  without 
any  offsets  or  stops.  The  curve  of  the  absolute  steam  velocity 
explains  itself. 

The  next  consequence  of  the  shape  of  the  pressure  line  is  that 
the  running  wheels  have  a  higher  steam  pressure  on  the  inlet  end 
than  on  the  outlet  end,  that  is,  the  whole  drum  with  all  the  run- 
ning vanes  is  thrust  axially  in  the  direction  of  the  steam  flow.  To 
balance  this  thrust,  balancing  pistons  are  used,  keyseated  to  the 
turbine  shaft  and  of  the  same  diameters  as  the  different  running 
rows,  and  being  connected  by  steam  channels  with  the  space  in 
which  the  corresponding  running  rows  revolve,  thus  balance 
each  running  row. 


Il^DEX 


Figures  refer  to  pages 


Absolute  and  gage  pressures,  60 
Absolute  pressure,  60 

vacuum,  60 
Action  of  slide  valve,  9 

of  steam  engine,  2 
Admission,  3 

line,  96 

pressure,  95 

steam,  125 

valve,  108 
Air  pump,  61 
Amount  of  vacuum,  61 
Angle  of  advance,  140 
Atmospheric  line,  90,  120 

pressure,  59,  60,  64 
Atomizer,  265 
Atomizing,  267 
Automatic  high-speed  engine,  13 

B 

Back-pressure,  91 
Balanced  value,  14 
Barometer,  60 
Bearings,  48 
Block  crosshead,  43 
Bore,  30 
Box  piston,  37 
Brake  constant,  128 

horsepower,  103 

Prony,  104,  126 

rope,  106 
British  thermal  unit,  57 


Center,  3 

Centrifugal  force,  212 
Classification  of  engines,  6 
Clearance,  4,  180 

volume,  96 
Commercial  cut-off,  396,  397 

29 


Commercially  dry  steam,  70 
Compound  expansion,  111 
Compounding,  111,  225,  239 
Compression,  3 
Computations,  126 
Condensate,  119 
Condensation,  111,  198 

of  steam,  242 
Condenser,  63,  225 

barometric,  249 

ejector  type,  253 

high  vacuum,  253 

jet,  247 

purpose  of,  240 

siphon,  248 

surface,  250 

Wheeler,  250 
Condensing  apparatus,  246 

engine,  7 

Connecting  rods,  45 
Consumption,  steam,  153 
Corliss  engine  cylinder,  34 

engines,  17 

valves,  17 
Counterbore,  30 
Crank,  47,  133 

and  eccentric,  position  of,  145 

angle,  141 

circle,  146 

pins,  47 

Crossheads,  42,  87 
Cut-off,  3,  176 
Cycle  of  steam  engine,  2 
Cylinder,  30 
Cylinder  condensation,  107,  225 

measuring,  114 


D 


Dashpot,  199 
Dead  center,  3 
Double  acting,  4 


287 


288 


INDEX 


Drum,  86 
Dry  steam,  66 

equivalent,  121 

line,  115 
Duration  of  engine  test,  122 

E 

Eccentric,  133,  143,  207 

circle,  146 

rod,  143 

strap,  203 

swinging,  173,  177 
Eccentricity,  133,  146 
Effect  of  heat,  57 
Efficiency,  mechanical,  106 

perfect  engine,  125 

ratio,  125 

thermal,  126 
Engine,  compound,  226,  236 

condensing,  241 

constant,  103 

Corliss,  145,  202,  241 

Cross-compound,  229,  233 

horizontal  4- valve,  122 

Leavitt  pumping,  122 

Mclntosh  and  Seymour,  122 

noncondensing,  241 

plain  slide  valve,  111,  171 

Rice  and  Sargent,  122 

Rockwood-Wheelock,  122 

simple,  226 

slide  valve,  222 

tandem-compound,  228 

uniflow,  112 

Westinghouse  vertical,  122 
Equal  leads,  161 
Events  of  cycle,  3 
Exhaust,  3,  120 

closure,  91 

lap,  134,  150 

steam,  125 

valve,  91 
Expansion  of  steam,  93,  227 

line,  96,  120 


Feed  water,  119 
Flywheel,  51,  211 
governor,  176 


Formation  of  steam,  64 

Frame,  27 

Friction,  255 

brake,  103,  118 
horsepower,  106,  130 
load,  118 


Gage,  mercury,  62 

pressure,  60,  63,  66 

vacuum,  245 
Girder  frame,  28 
Glands,  41 
Governing,  211 

Corliss  engines,  20 

high-speed  engine,  15 

steam  engines,  12 
Governor,  centrifugal,  221 

Corliss  engine,  208 

Hartnell,  218 

loaded,  218,  220 

pendulum,  212,  215 

Proll,  218 

Rites  inertia,  222 

shaft,  20,  221 

throttling,  182,  213,  217 

Watt,  214 
Gumming  test,  260 

H 

Heat  of  the  liquid,  65 

units,  57 

Heavy  duty  frame,  28 
High  pressure  cylinder,  212 
Horizontal  engine,  6 
Horsepower,  58 

brake,  103,  118 

constant,  127 

indicated,  102 
Hunting,  216 


Impulse  turbine,  274 
Indicator,  79,  81,  92 

diagram,  123,  135 

spring,  99 

Inertia  governor,  222 
Initial  condensation,  111 
Inside  admission  valve,  141 
Interpolation,  68 


INDEX 


K 

Knock-off  lever,  208 

L 

Latent  heat,  65,  67,  69 

of  evaporation,  65 
Lead  of  valve,  139 
Links,  movable,  85 
Locomotive  pistons,  39 
Lubricating  external  bearings,  261 
Lubrication,  255 

cylinders,  268 

external  bearings,  261 

pistons,  268 

steam  engine,  261 

valves,  266 
Lubricators,  hydrostatic,  263 

M 

Marine  engine,  26 

cylinder,  36 

pistons,  39 

Mean  effective  pressure,  98,  100 
Measure  vacuum,  61 
Mechanical  efficiency,  106 

equivalent  of  heat,  58 
Mercury  column,  243 
Metallic  packing,  41 

N 

Noncondensing  engine,  100 
Nonreleasing  Corliss  engine,  23 
Nozzles,  280 

Numerical  relation  between  heat  and 
work,  58 


Oils,  testing,  260 

Open  rod  construction  link  motion, 

189 
Over-governing,  216 


Pantograph,  reducing  motion,  89 

Partial  vacuum,  60 

Parts  of  steam  engine,  5,  27 


Perfect  engine,  efficiency  of,  125 
Perfect  vacuum,  60 
Piston,  37 

dashpot,  199 

displacement,  94,  116 

locomotive,  39 

marine,  39 

position,  144 

rings,  38 

valve,  14 
Planimeter,  98 
Port  opening,  148 
Power,  58 
Pressure,  59 

absolute,  60 

atmospheric,  59 

back,  240,  241 

exhaust,  123 

gage,  60,  90 

maximum  admission,  96 

terminal,  153 
Properties  of  steam,  66 
Pump,  compound,  131 

small  direct  acting,  135 

Q 

Quadruple  expansion,  226 


Radius  bar,  194 
Ratio  of  expansion,  94 
Reciprocating  parts,  1 
Reducing  motions,  87 
Re-evaporation,  109,  111 
Regulation  of  speed,  11 
Release,  3 
Reversing  gears,  183 

Woolf,  197 

Rocker  arm,  138,  159 
Rods,  piston  and  valve,  268 


Safety  cams,  209 
steam,  69,  71 
Second  admission,  235 
Sensible  heat  of  steam,  65 
Setting  a  slide  valve,  161 


290 


INDEX 


Shaft  governor,  220 
Shifting  eccentric,  171 
Simple  engine,  6 
Single  acting  engine,  4 

eccentric  valve  gear,  200 
Slide  valve,  133,  164 

action,  9, 

engine,  8 

Slipper  crosshead,  44 
Specific  heat,  58 
Speed  regulation,  11 
Stability  of  governor,  214 
Steam  consumption,  119,  122,  153 

engine,  efficiency  of,  123,  124 

jacket,  111 

lap,  134,  150,  192 

line,  90 

meter,  119,  123 

pressure,  60 

superheated,  262 

tables,  65 

turbine,  Curtis,  282 

turbine,  operation  of,  271 
Stephenson  valve  gear,  195 

link  motion,  184 
Stuffing  boxes,  40 
Superheated  steam,  70,  111,  121 
Surface  condenser,  119,  122 

T 

Temperature,  57 

absolute,  125 
Test,  acid,  261 

flask  and  fire,  260 

gumming,  260 
Total  heat,  65 
Tram,  159,  202 
Trick  valve,  166 
Triple  expansion,  226 
True  pressure,  60 
Turbine,  DeLaval,  280 

impulse,  285 

Westinghouse-Parsons,  286 

U 

Uniflow  engine,  112 
Unit  of  heat,  57 
of  power,  58 
U-tube,  62 


Vacuum,  59,  61,  243 

gage,  63 

measuring,  243 
Valve  action,  3 

auxiliary,  180 

balanced,  177 

Ball  telescopic,  168 

circle,  151 

Corliss,  17,  198,  202,  267 

diagram,  Zeuner,  146 

displacement,  143,  146 

gear,  159,  183,  193 

ideal  piston,  170 

inside  admission,  142 

lead,  148 

Meyer,  179,  182 

multi-ported,  167 

piston,  169 

poppet,  267 

rod,  138 

setting,  157 

slide,  266 

steam,  201 

stem,  156,  158 

straight  line,  167 

straightway,  83,  84 

three-way,  84 

throttle,  211 

travel,  133 

trick,  166 

with  lap,  136 

without  lap,  134 
Vane,  275,  282 

stationary,  286 
Vapor  pressure,  245 
Variable  loads,  118 
Viscosimeter,  Saybolt,  260 


Walschaert  valve  gear,  184,  191,  195 

Wet  steam,  69 

Wing  crosshead,  42 

Wire  drawing,  167 

Woolf  reversing  gear,  184,  196 

Wrist  plate,  198,  207 


Zeuner  valve  diagram,  202 


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